Well TrueCrypt and similar (eg. Bitlocker) operate at the file system level. I use Bitlocker on my laptop since it has a TPM module and allows for single sign-on (I don't have to put a password to boot the computer and another to login to my computer), but each have their advantages and disadvantages.
One reason why someone might not want to use a file system level encryption suite with the Intel 330/520 series is because they use the Sandforce SF-2200 series controller. Because Sandforce controllers owe a lot of their performance advantages to performing realtime data compression, dealing with encrypted data (which is very nearly incompressible) will cause quite a large performance loss. Anandtech's bench results
help illuminate the difference you will see when enabling TrueCrypt or Bitlocker.
Lastly, performance wise, the encryption provided by Intel SSDs is completely free. If you have to rely on your CPU to encrypt data at the file system level, you may see higher CPU usage and lower read/write rates. Then again, if you have a relatively recent CPU that supports AES-NI, this point is moot.