I agree with JCL. The demands and environment (heat, lack of combustion by products, etc) is much less in the transmission and differentials.

I just changed the diff fluid on our 98 323 after 193,000kms (120,000 miles) and it was still very clean.

I think fluids and tolerances are getting better and better. If you are not hard on your equipment I don't think you need to worry. Just be sure you are not running in the 'extreme service' categories.

having said all that, it is cheap peace of mind if you intend to keep the vehicle for a long time.