I believe that with proper wheel maintenance and landing from jumps smoothly this issue becomes less of an issue.
If they do get bent, I'm sure that the sponsor will help out to replace them.
The reason companies offer sponsorships to teams, is so the teams using their products promote and help to develop/improve their products. No company wants to be known as the "bent rim association" or "grenading piston foundation" etc.
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I have a 1990 YZ250 with the stock rims, it was origionally ridden by a regional pro, then sold to one of his less than talented friends that negleted and abused it for the next 6 years in a gravel pit in Michigan.
I bought it in 1999, tightened the spokes, trued out a little wobble, and keep up with the wheel maintenance. I have put at least a thousand hours on that bike in the tight rooted southeastern woods.
When I raced BMX as a kid there were a couple of guys that kept getting new/different wheel sets every month, and complaining about this rim sucks or that rim sucks, cause they bend, or the spokes come loose. I rode the same Araya 36 spoke rim/Suzue High flange loose ball hubs from 84-91, on different frames, I replaced the spokes twice, the freewheel sprocket 4 times, trued out several pretty big wobbles, and greased the loose ball bearings every month. I worked mowing yards everyday for the whole Oklahoma summer to buy that wheel set, and when it came time to get them I didn't figure in the shipping fees, so I had to settle for the loose bearings instead of the preferred and dreamt about nightly ,sealed bearings. The guy I sold the bike they were on to, still has it, with the same wheels.
What I came to realize was that those guys never adjusted thier spokes, or fine trued the rims. They also had a bad habit of landing hard or sideways and loosing to me every weekend.
That new style of MX riding the kids do now, with that speed scrub little whip thing they are doing over jumps, and landing it "not so straight" the majority of the time combined with a lack of proper and regular wheel maintenance is the reason for bent rims.