Nothing wrong with taking the extra time and polishing the bike up. Just hosing the dirt off, is not allowing to do a good inspection of your bike. These are dirt bikes and most of you ride them hard (excluding me of coarse). That causes wear and tear and if you just hose them off and put them away, you will not see the little parts that are broken or have cracked. Water will sit and rust some parts. Taking the time to polish the exhaust pipe will allow you to inspect the pipe mounts for broken mounts or cracks in the mounts.
Or you can ride it until crap starts falling off on the trails and it cuts your ride and your friends ride short....
As for me, I keep my bike super shinny because it needs to look good when sitting on the stand all day as Trav mentioned above. I like when people think I am really fasts because my bike looks all shinny and new. Boy are they disappointed though when I occasionally take it down...