Depends on how severe the dropped fetlocks are. The ligaments stretch over time if not attended to immediately, and can eventually snap.
I had a broodmare agisted on my property last year, with dropped fetlocks. She was unable to carry a foal in the end because of them, although was not real sore so she was keeping a retired gelding company.
However, they ended up dropping to the point where one day one of her ligaments snapped, and the associated fetlock was on the ground, thus she was euthanised that day.
I will not look at a horse that has suspect legs, a little pigeon toed etc. I can deal with, but because the horse's body is so heavy, on what are proportionately very small, weak legs, I will not take the risk purchasing a horse with dodgey legs.
Typically, I have still had problems in the hind end with various horses, even after pre-purchase exams and having correct conformation. My current horse came up totally sound and has a good hind end conformation, and now that I have started asking for a small degree of collected work, he's come up dead lame and sure enough has a bone spur in his hock that may or may not mean the end of his career.
I figure, and yes there are the lucky cases where leg deformaties do not cause a problem (I competed very successfully on a severely pigeon toed mare), but I will avoid hind leg problems at all costs, to try and minimise the occurance of such injuries and long term problems.