Curious what purpose you see in it?
If we're going to have classes, demarcating their boundaries is part of that. Sure, not every character of a certain class is going to have the exact same skills (even aside from backgrounds), but if classes are types (and there's at least an argument they are) then certain skills are, well, typical.
Someone earlier (I think) also said something about the limitations making it easier for beginning players to build a character, and there seems to be some wisdom there. Even that aside, some people find inspiration more in limitations than in ... the opposite. It's like a drummer choosing not to have any cymbals on their kit, or a music producer choosing to record on tape rather than a computer (differences in sound aside in the latter).
Also, if certain skills go with certain classes, it serves a purpose similar to niche-protection, in that it gives all the classes something out-of-combat to be good at, or at least do (and I believe characters should have something to do in-combat and out-of-combat). I think is also makes the classes a little more coherent.
With the flexibility in backgrounds, you can make a character with at least one idiosyncratic skill easily enough; two if you go completely free-form. It's not so limiting that I at least feel straghitjacketed. Obviously, experiences differ.