I read in one thread on some site (it may have been a sword site) that some of the members of the site were trying to figure out why this was done (bending the quillon). After testing out different scenarios they decided it was not to give a better thumb-hold. If you have ever fenced saber you may have been taught to place your thumb along the grip pointing in the direction of the blade and then wrap your fingers around the grip. In this way you can make cuts by flicking your wrist. If you have an old broad sword from the 17th or 18th century you might notice a thumb ring next to the grip. Instead of laying your thumb along the grip in the direction of the blade you insert it into the ring to give you more leverage. What the sword testers of the message board decided was that when you bring the sword down in a cut, as the blade makes contact with the target it begins to slow down quickly. Meanwhile your hand is still moving as if the sword is continuing at speed in it's arc. What happens is that your hand and the quillon make an uncomfortable collision. By bending the end of the quillon down they could reduce or eliminate that collision and save their hand for something else, like shooting a revolver. Since I was not involved in this bit of experimental archaeology I can't verify the results but it would seem to indicate that those swords with a bent quillon belonged to troopers who used their sword or had used it in the past, while the swords with an unmodified quillon belonged to troopers who shot first and slashed later.
There are some cavalry sabers in the study collection I work with that have bent quillons. Since it was a field modification, for whatever purpose, I leave them as they are. There are some that have had bent quillons that someone has tried to straighten, but you can always tell and it usually looks worse with the quillon being bent up a little or developing cracks along the bend line. I leave these alone too because experience has taught me that many times you make things worse when you try to fix them.