You wouldn't donate the crappy secondary reg, that's the whole point of primary donate. The drawback of having a "crappy" secondary is that it's the reg YOU will be breathing on when you donate the better primary that you were just breathing on. So switching to a long hose setup now is actually making better for all of your buddies in the water, but depending on the condition of your "crappy" secondary, you might be making it worse for yourself. However, when you dive with a primary donate setup, you have an even bigger incentive to make sure both of your regs are in good working condition and breathe well.
PS. I don't know the regs you have, but you don't need a top of the line reg to have something that breathes well. The bottom of the Apeks line XTX40 breathes plenty fine if it's tuned and maintained well. Can't speak for anything Mares.