The problem with just adding in "well, I want to be able to just call any old arbitrary thing a skill if I feel like" is it really DOES put a big monkey wrench in things. Martial Practices sort of have the same problem. As soon as you say you CAN get 'boating' as a skill, or 'potting' or whatever INSTANTLY any PC that didn't make that choice must logically be less competent or in many cases where something requires real SKILL at doing it, like throwing a 3' vase, all other characters really logically have to be incapable of even trying it without burning a skill slot. At least backgrounds are 'free' since they have no major mechanical implications that matter beyond the one you take to get a bonus skill/+2.
It is even worse than that when adding skills. It doesn't even really simulate very well the thing it is trying to simulate. Characters in some fantasy medieval environment can do all kinds of things well. (This is another area where Hero tries, but falls short, with its "Everyman Skill" rules that give a roughly 25% chance to succeed on any skill that most everyone can do.)
I believe it was Chesterton that said something along the lines that there were some things we expect a person to do well, or not at all. However, there are a lot more things we expect a man to do adequately, even if he doesn't do them well. The list, of course, varies with people, eras, and environment--though I think it is sound enough as a rule of thumb.
The canonical modern example for do well or not at all is often "brain surgery". No one wants me tinkering with brain surgery. With music, however, I have at various times played piano, baritone saxophone, guitar, sung, and composed music--well enough to play in a group in public and not make people embarassed on my behalf. I've even done a few solos. Much of it has fallen by the wayside over the years, but I still play piano for enjoyment. Unlike brain surgery, dabbling in music is something I think most people would consider a good thing (or dance or painting or whatever one does like that).
While I'm well short of Heinleins' famous list, I can also cook a good meal in several styles, wire a house, do simple but quality woodworking, write a sonnet, and a bunch of other things like that. All of them are the result of many hours spent in serious pursuit of excellence. In some cases, I do them better than
some professionals, even though I have never done them for a living. (This is more a testament of the low quality that will suffice for a person to "succeed" at a career, than my high skill, but you get the idea.)
None of that matters diddly squat in a contest. Because there are professionals (and amateurs so dedicated that they might as well be) that can run rings around me. I'm not even in their league. And while most people that set their minds to it can do a lot of different things
well, there is a limit for what one can
master--age and physical fraility, if nothing else.
Skill rolls that creep up slowly don't even began to reflect this. So adding a bunch of skills to a list is very misleading, especially skills with low value that creep up incrementally. In reality, even fairly clueless people pick up all kinds of highly useful and productive skills almost by osmosis--by living in an environment. Go be the helper in a medieval forge, and after a few years, you wouldn't have PS: Blacksmith +5. You'd have solid skills with metal, carpentry, animals, small business management, leatherworking--and a few other things that were needed locally just becaue you were "handy". It is almost more accurate in a game model--if you want a real simulation aspect--to list what such a character can't do, rather than the other way around. Guy the Blacksmith never learned basic heraldry or any stone masonry. It's just a quirk of his character and interests.