I guess it really depends on your driving style&location. Look at it this way: I never had an issue with the arms so why upgrade to RPM?
I only drove the Muggy in the sand, and that stuff really gets everywhere. To me $100 doesn't sound bad if it'll keep sand away from the electronics, center diff&gears. You could use tape to cover up the holes, but a big truck like the Muggy deserves a big chassis:
http://www.rc-monster.com/forum/show...&postcount=206