I agree with you, Pam.
Besides that, if the Sims 3 came already with all the expansions and stuff all at once, then I think in the long term the game will get boring because there will not be anything refreshing and new, like what they do now.
Okay, it would absolutely be awesome in the beginning and I probably would play it around the clock (or just 20/7 in case I decide to sleep), but after some time in a rehab clinic without any new expansion or stuff pack to look forward to, I would probably find another hobby.
So the expansions are good, in my opinion. It is the only way (at the moment) to let games be interesting in the long, very long run.
Besides MMORG's of course.