trans service interval

I recently bought a 2001 KDX200 and quickly joined this forum. I've gotten plenty of good tips by searching the forum here. I particularly appreciated the tips on making the bike better for guys over 6 feet tall and over 200 lbs., and the need to service the KIPS valves(I would not have guessed they were a maintenance item). One question I couldn't find an answer to is; Why do so many of you service the trans oil so frequently? From the posts I've read some guys are changing it every few rides. I could see doing it every couple of rides if you were getting a lot of water in there, but you'd see that show up quickly if that was happening. What is contaminating the oil so quickly that it need to be changed constantly? It can't be wear metals from the trans. If it was, it seems there'd be nothing left of the transmission before too long. Are the wear particles from the clutch abrasive and we need to get them out of there?
Keep in mind I'm not an experienced 2-stroke owner. Most every other bike I've had was a 4 stroke and the trans oil got changed with the engine oil, except for my '78 Yamaha DT-175. The only time I've changed the trans oil was when I've submerged it.
Any and all replies welcome, even if you're just going to tell me to "Just shut up and do it. It's inexpensive and easy to do."
Thanks in advance.
Joe

Thanks for the reply. That seems simple enough. I never thought that aluminum clutch plates were a problem, but I'll definitely keep an eye on it. Good point about the filings accumulating on the bottom of the trans.
Joe