I agree that CLR for a simple, relatively flat pseudorandom number is overkill.

However, the original poster is specifically using a function designed to generate crytographically secure random numbers, which is an entirely different ballgame. crypt_gen_random uses the older (Pre-Cryptography Next Generation) API, though I can't tell as easily what API the .NET RNGCryptoServiceProvider uses, though I suspect it's also pre-CNG.

For any cryptographic use (encryption keys, random passwords, etc.), the quality of the random number source is very important to the final quality of the cryptography.