I think the reason for excitement is pretty simple really...
Boston have been championship contenders since Ray and KG came over to form the big 3 in 2008. That same year we won the title.
Every year since then, our starting lineup has been one of the most dominant in the league, but two things have held us Celtics back from getting that second title:
1. Injuries
2. Bench production
Since 2008 the only genuine sixth men (i.e. people who could actually lead the bench and produce) were Glen Davis and Jeff Green.
Glen Davis could give us much as a sixth man because he kept having to fill in the starting lineup for other injured guys (KG, Perkins, Shaq, JO). Last year Jeff Green and Bass were supposed to be those guys, but Green was lost for the season along with JO, and Bass had to move to the starting lineup to replace JO - this left us with once again no scoring off the bench.
When Ray Allen was coming off the bench briefly last season, it was the first time in years that we had a legitimate 6th man who could put points on the board - then he got hurt, and now he's gone.
Every season without fail, Boston's starters have had to drag their pensioner butts around the floor for excessive minutes trying to make up for the scoring drouts that occured every time the bench came in - and ultimately it was always out downfall.
Now this year we added Terry and Green - two guys with the potential to give us 15-20 points on any given night. That was great, but it left us with two problems:
1. Uncertainty on when Bradley would return means Terry would have had to start, leaving us with again only one scorer (Green) off the bench until he returns
2. Given Boston's history with inuries, if one or both of those guys get hurt then we are back to square one - no scorer off the bench.
Courtney Lee is huge for us because he gives us that third guy as critical insurance.
1. He gives us a guy who can start at SG until Bradley returns, allowing us to continue to bring two strong scorers (Terry and Green) off the bench
2. He gives an extra scorer as insurance who can fill in if worse comes to worse and eiher Terry or Green gets hurt.
Worst case scenario, he gives us the type of insurance that could be the difference between a title and an early playoff exit.
Best case scenario he gives us a third consistent double figure scorer who can come off the bench and put points on the board.
How many teams in the league can honestly say that they have three legitimate double figure scorers coming off their bench? Not many...
Plus that's not even taking into account Sullinger (as he's unproven) and Wilcox (as we don't know what shape he's in).
What it means is that when we are healthy Pierce, KG and even Rondo can take a breather while out second unit comes in and not only maintains, but actually extends out lead.
Then on top of all that, he gives us a bigger guard (whih we so depserately needed with Terry and Bradley at SG) who can defend bigger and stronger guards like Wade, Iguodala or Harden come playoff time.
All of this flexibility listed above all but vanishes without Lee - it all breaks down.
So, that's why we are excited over a guy who is nowhere near as bigtime as out excitement levels may suggest. It's not about who he is, it's about what he gives this team.
Take a look at the Heat for example, and the did the complete opposite. They picked up Ray Allen and Rashard Lewis - two big time veterans (or at least they once were), but two guys who only really have skills in three-point shooting...an area Miami already has covered with Miller, Battier, Chalmers and Jones. They added plenty of fame, legacy and talent but it's all redundant - they didn't fill any of their big holes (PG, C).
We got a more or less 'under the radar' guy who fills exactly the hole we need filled. That's something that's worth getting excited about!!