Thomas Asks: Are back squats necessary for weightlifting and in what situation should a lifter only program front squats? 

Greg Says: Technically nothing is necessary for weightlifting other than the snatch and clean & jerk—everything else is supplemental and intended to improve those two lifts. That said, exercises vary in their utility and importance.
 
The back squat is a very valuable lift—it’s going to build the most “general” strength for the lifter. It builds not just the legs and hips, but also the trunk, and allows the most loading for the squat possible. However, there are times when the back squat isn’t possible for a given lifter due to an injury or other condition that prevents its performance at all or with loading significant enough to be meaningful. It’s in these cases that the back squat can and should be abandoned—really there isn’t a choice anyway.
 
Before leaving it behind, though, do some investigation and try figure out the source of the problem and if it’s something that can be resolved—it’ll be well worth the effort if it can be. It’s not normal to be unable to back squat, so if you can’t, there’s a problem to be diagnosed.
 
If you have to quit back squatting, you can replace it with front squats, but you might want to add a bit more hip work in the form of deadlift or pull variations, good mornings or RDLs/SLDLs. You can also try less common variations like trap bar deadlifts on a riser to simulate more of a back squat position and range of motion.