Tranny is mentioned for 20K severe use service schedule (I figure that is half or less of normal usage.) Changing the filter, although more costly than doing just the fluid, is way important since it catches the normal sluffing off of clutch particles and debri - it is a very fine filter material. If it clogs, it is a bad thing.

I flush my fluid every 15K and do a filter change every 30K - new tranny's are $3500+ Like they say about oil and filter changes, pay a little now or a lot later.... Can't change ANY fluid too often IMHO.