Ok first of all where does Avery Bradley is better than Ray Allen stuff come from? I would be utterly shocked if there is a single person in this world who knows even the smallest amount about basketball believes this. Let's clear out some misconceptions right now.
Manu Ginobli, James Harden, Jason Terry are better players respectively than Daniel Greene (who?), Thabo Sefolosha, Stevenson (last year for Dallas) All those guys close out the games for their respective teams, not their starters.
Why then do these players, and Ray, come off the bench? What possible reason can there be to start a clearly inferior player? There are a number of reasons.
1) Bradley (great kid and super talent) only looks good BECAUSE he plays with the starters. Ever notice how NONE of his threes are contested, while Ray has to run a marathon to get open for a half a second? That's because defenders are more concerned with Rondo, Paul and KG.
2) Rondo has more opportunities to score. With Ray, Paul and KG on the floor, Rondo defers way too much and doesn't look to score. He will feed Bradley if he's wide open, but Bradley has not had one single play run for him in his career according to Doc. This simplifies the offense.
3) Paul Pierce is averaging over 20 ppg for the last couple of months. KG is scoring as much as he ever did as a Celtic. This is because Rondo is now on the floor with only 2 of the Big 3 at a time. Starts the game with KG and Paul so they really look to score. KG leaves and Ray comes in, Paul leaves and KG comes in, and Ray gets his rest and the Big 3 close out the game. That means he the offense focuses more on each of them when they are in the game instead of having an extra scorer running around wasting his energy.
4) Ray is not the only starter who plays with the bench. We have NEVER seen Ray alone with the bench. Because of his 5-5-5 time KG comes in and plays with the bench unit. In fact, when Rondo sits, our offense runs through KG.
5) Ray either is guarded by a second unit guy, or a tired starter who doesn't want to chase him around screens. This hopefully will make Ray a better scorer. Doc has explicitly stated they moved Ray to the bench because they need his scoring and he doesn't get the same opportunities when he plays with the starters because we have too many weapons on the floor.
6) By spreading the Big 3 over multiple units, we no longer have a unit that has no offensive weapons. At least 2 of the big 4 will be on the floor at all times, most of the time it's 3 of the big 4.
7) Ray coming off the bench has NOTHING to do with his skill set. If Jeff Green had a breakout year instead of Avery Bradley, Pierce would be coming off the bench. It's only Ray because Bradley can't play any other position.
The Big 4 is a dominant group. They outscored the opposition overwhelmingly last season, but our bench kept losing all their points. Starting Bradley means our starting group is not as dominant but it does utilize each of the big 4 more over the course of the entire game. And we no longer have to worry about the bench losing all the advantage the starters build up.