Not to mention that if the rabbit is outdoors, the owner is less likely to notice the onset of medical problems such as GI stasis, diarrhea/cecal problems (which can also lead to fly strike), urinary issues, etc. Especially with GI stasis, the early it is treated the more likely it is that the rabbit will survive.
I've never heard of a rabbit breaking it's leg by thumping before, but I do know of one that got a concussion when an oppossum scared him, and I've heard horror stories about what happens when a neighbor's dog gets loose.:sad: Or worse, the neighbors kids!>(
I don't know how guinea pigs are with heat, but rabbits have trouble dealing with temps much above 80F. 90F has been known to cause heat stress/stroke. They're meant to be underground in their burrows during the hot day.