I am feeling the urge to stick up a bit for the crossbreds here... The comments about this mare's faults are valid...
But without seeing either parent it is impossible to say for certain which faults came from which parent.
I know TONS of registered QH's which are sickle hocked, and straight behind... plenty are also tied in and back at the knee too. It is as possible that she inherited all her weaknesses from one parent or the other... Rather than sticking a breed generalization on faults, simply acknowledging the fault's existence is probably good enough.
Maybe I have been lucky, but I can't say I have seen too many sickle hocked Percherons compared to how many stock horse breeds I have seen sporting that fault (and all of this mare's leg faults, actually). But that is rather besides the point, I guess.
I actually have nothing against crossbreds... I worked with outfitters for awhile and we used, primarily, QH x Drafts (Belgians and Percherons) for packing, driving and carrying around green riders. Most of the ones we had were no more riddled with conformation issues than any other horse. (A couple exceptions... But you do see that within registered breeds as well) We had some AMAZING looking animals in our strings... Stocky bodied, medium height, heavy bone, large feet and nice square profiles. The breedingstock for the working strings was carefully balanced. None of the QH stock was light in bone, or small in hoof, and the drafts were chosen carefully to not be too heavy. Conformation faults were minimal, and never doubled up in both the mare and stallion. Again, I sort of digress though.
I do, totally, agree that this mare does not need to be bred. To me, the idea behind any breeding is to better the parents, with a specific purpose in mind. Looking at this mare, I feel, she is a nice enough looking lady, which will make a lovely pleasure mount (as in not heavily competed or hard used... Not the actual discipline, just to clarify that comment) but... She does not stand out as a cross which would be a candidate for breedingstock, regardless of which way you wanted to breed back to.
There are some very successful crossbreed programs... The reason they are successful is because the breeders are careful to not breed combinations of faults. If the worst thing this mare had going for her was that she was light in bone, that would be one thing... But to be light in bone and back at the knee, then tied in behind the knee, then post legged and sickle hocked.... Well... The odds of the resulting foal having combinations of faults as well is just too high to go through all the breeding risks... IMO