PF/C's with bad ATH just get smoked playing defense and way way under-perform on offense. Put another way, on offense they shoot like crap most times and rarely rebound well (not a 100% thing, but it isn't pretty most times) and on defense they foul a LOT, even if they have stellar defensive ratings. Understand too that all this is in DIRECT corelation to the ATH ratings of the other team/player they match up with, which is why a low ATH guy *can* have a very good game from time to time too.
End of the day, you have to work with what you have, no amount of planning or rearranging is going to make a bad player a good one. I generally try to put the highest ATH/REB combo at C and the next highest at PF, unless he's SUPER slow (like 1-12ish) then I look to the 3rd highest and play him unless his ATH/REB is drastically inferior to the #2 option.