Thunderbird has limits, it will one day crash and eat your mail, it can get locked with attachment hacks, and the spam filter is very basic and easy to beat and it has memory limit on the amount of spam it can learn.
I get 1500+ spams daily, every day.. easily. I have tried 25-30 different email clients and spam filters to end up using
http://www.spamfighter.com/ with outlook express.. I get about 2% spam now, compared to 30-40% with Thunderbird.