Ray Allen was coming off an ankle injury that lowered his value somewhat and he was also 32 years old as people have mentioned. That's why he was available for what some may consider a low price. I tend to think regardless of the perceived weakness of that draft, it was a fair trade. It was a real possibility at the time that he wouldn't age as gracefully as he ended up doing.
Also, Ray Allen was a much more complete player than Thompson is at this point. I think some Boston fans forget how athletic and versatile he was when he was younger because he lost some parts of that with old age (you could still see it in flashes though). I'm not badmouthing Thompson, just reaffirming how good Ray was.
The closest parallel to Garnett is Davis in terms of big men with that level of impact on the game. There isn't really another big man that is close. The other big names right now are on a tier below, like Cousins, Aldridge, and Love. Unfortunately that's just how rare it is for someone to come into the world that tall and still possess elite athleticism and coordination. From that era there really was just Duncan, Shaq, and Garnett whose ability was at that special level (Nowitzki and Gasol are somewhere behind them in the next tier).
OK, so we established these are special players who rarely become available. Does that mean we shouldn't pursue stars on that second tier? It's a tough question, because you don't know for sure who is going to be available, and they might not even come. Garnett almost didn't come, after all. LeBron didn't go to a lot of teams lining up with that hope.
Personally I don't see the Celtics being able to make a quick turnaround a la 2007. I don't view Rondo as being on the same level Pierce was and he doesn't seem like a guy superstars are clamoring to play with, unless it's Rondo going to THEIR team. The "close friends" we DO hear about like Josh Smith are on the lower rung of star. Put it this way, Rondo and Smith trading texts to team up does not exactly bring up memories of LeBron setting up the three-way Skype conference with Bosh and Wade.
It's a painful thought to imagine the Celtics best option might be to continue rebuilding a year or two but it may be the most prudent course of action.
Oh, here's another situational difference between Pierce and Rondo: At age 28 (2006 offseason) Pierce signed an extension with the Celtics even though they had just finished a 33-49 season and the second best player on the team was a 21 year-old Al Jefferson. Rondo is in a similar situation this offseason but the CBA dictates he wait until next year for the maximum possible extension. This means the Celtics don't get a grace period where their star pouts (as Pierce did) but they are free to not make any desperate "win-now" moves.