Spydie invented the hole as a one-hand opening device, and if they didn't they were at least the first to be smart enough to patent it, which forced all other knife makers to pay them a royalty if they put a hole in the blade for this purpose...
Now the hole does constrain the blade design somewhat... There has to be a place to put it... Makes sense... All the other schemes that are devised to support single-handed opening, bumps, slabs, and what not, allow their makers to avoid paying the royalty to Spyderco, but also permit a much wider degree of blade profile design because most of these alternatives are much smaller than the hole and can be mounted on almost any type of blade, even very narrow ones...
That being said, the hole does appear to be the easiest and most versatile one-hand-opening accoutrement... They are equally available to the right or left hand, can be easily used with gloves and can be easily gripped between thumb and first finger to flick the handle down and open the knife that way if you so prefer that opening style (possibly not good for the knife over all, but that depends on a lot of other things). My all-steel model Spyderco Delica is beyond any doubt the smoothest opening knife I own (out of dozens of folders), some costing 10x more!
My $0.02