The next time I see Rondo pressuring the ball would probably be the first time since his rookie year. Likewise, I've never seen the team apply full-court pressure for the whole game, and I think it's a terrible idea -- it's way too much energy to expend, it can be handled with a single back-court screen, and it's just not worth it to risk having your smallish guard run full speed into a 250 lbs guerilla while hustling up the floor. There is a reason why most teams in the NBA don't do that.
Oh, and it also means that one of those two will have to be on the floor at all times, which actually minimizes the gains from the alleged synergy (and may force Bradley in the PG spot at times, too).
Also, I don't think anyone has ever argued that Bradley plays bigger than his size on defense (as a matter of fact, he probably plays smaller than his size because of his quickness, if that makes any sense).
The sample size with Rondo and Bradley on the court together has been relatively small over the last few years. This is Because Bradley has spent a large amount of time either riding the bench (earlier in his career) or injured (later in his career).
Two seasons back when Bradley was first given legit playing time by Doc, the Rondo-Bradley pairing was dominant defensive, and Bradley statistically had one of the best on/off numbers in the entire league. He seemed to trive when playing with Rondo for exactly the reasons mentioned above - he would get a ton of easy layups off cuts, he was hitting a good percentage of open jumpers from midrange and from three (he shot around 40% that season, and has been around that figure much of this season too) and defensively the combination of Bradley's full court "in your face" pressure and Rondo's ability to knock away balls and get in the passing lanes meant absolute havok for opposing teams.
That backcourt combination was a big reason why we we made it to the ECF against Miami that season with Pierce and Ray shoting a combined 35% or so from the field, and one of the worst benches in the league (Mickael Pietrus was our 6th man and Ryan Hollins!). Unfortunately Bradley was deactivated just prior to the Miami series, a series in which we gave the highly favoured Heat a hell of a fight...and I personally believe the series woudl have gone very differently if Bradley had been playing and healthy.
Last season Bradley wasn't the same effective shooter on offense, but he only played half a season and obviously struggled after returning from surgery on BOTH of his shoulders. Only a handful of games after he did return, Rondo was lost for the season.
I honestly believe that Avery has a greater impact offensively when he's playing alongside Rondo, because all of his shots come so much easier. I also think Rondo is a better player defensively when he's playing alongside Bradley, because Bradley's ability to pressure the ball allows Rondo to get some preserve some energy. I honestly think one of the big reasons why Rondo doesn't pressure the ball as much as he used to is because he uses so much responsibility on offense that he doesn't have the energy to apply constant pressure defense at the same time. With Bradley pressuring the opposing PG, Rondo can play off the ball on defense at times and pressure the ball in the half court, which takes up far less energy.
As for one of them being on the court all the time - that was pretty much already the case and almost always has been when they have played together. When Rondo sits it will probably be Pressey + Bradley and when Bradley sits it can be Rondo + Bayless. I think you'll find there will be very few moments where both Bradley and Rondo are on the bench at the same time, once Rondo is back to full health.
I think it's very fair to say Bradley plays bigger than his size on defense. Dwyane Wade is considered by most to be a 'big guard'. He's listed at 6'4" but is really closer to 6'5" with shoes, and at around 220 lbs he has the strength of a small forward. Bradley (when healthy) has given Wade fits in the past defensively, despite giving up both size and strength. Yes he will still struggle at times against really big guards like Joe Johnson, but against reasonably big guards (Wade, Kobe, Tyreke, etc) Bradley is quite capable of holding his own.
Likewise Rondo. I've seen Rondo defend guys as bit as 5-6" bigger than him. I still have that image in my head of Rondo defending Lebron in the playoffs a few years back both on the perimeter and in the post, and giving him more than a bit of grief. Sure you'd never want to have that matchup and Lebron will obviously domimnate him most of the time, but the point is that Rondo is pretty capable of defending bigger players when he's healthy, and can even hold his own in the occasional case where he is way overmatched.
Again, the fact that Bradley plays best when not forced into the PG spot isn't the same as saying that the Rondo-Bradley pairing is a great fit. A more conventional SG who can hit a three, shoot over defenders, and perhaps not completely fall apart if they have to finish a contested layup would be better. Rondo-Bradley wasn't a very good offensive pairing last year (Rondo-Lee, on the other hand, was).
A more conventional SG won't give you All-defense calibre defensive pressure like Bradley does. Everybody sees matchups that they struggle with at times (such as Felton killing Bradley in the playoffs last year) but with Bradley those matchup problems are pretty rare. More often than not he dominates his assigment defensively and makes there life far more difficult and last I checked his defensive stats (opponent points allowed, opponent FG%, etc) indicate this. Outscoring your opponent is the name of the game here - whether you do that by superior offensive or by superior defense makes no difference. If anything defense is preferred because every good offensively player has days where shots just won't fall, but good defense is an effort area that a player can perform at consistently.
A more traditional scoring SG might give you 2 or 3 PPG more than Bradley, but they might also allow their opponent to score 4 or 5 PPG more than Bradley. Fact is that Bradley is a solid enough shooter that he has to be defended, and opponents respect him enough that they won't play off him the way they do with Rondo - if they do he has the ability to hit the shot with some consistency and can make the defense pay. If you have a guy who can consistently play elite defense and at least hold his own on offense, then you're doing well.