In many situations you do know what you're fighting in advance though. If I'm working on a contract for X monster, I'll check the bestiary to see what it effective against it and check that I have the appropriate oil, decoction, and potions. I'm only level 18 right now and have been spending skill points so as to not have more skills than I have slots right now, but later on I would adjust this too, if needed. Most story line questions result in fights versus humans so I'd use Hanged Man's Venom before I think there will be an encounter. Other quests often provide clues if you find them with the witcher senses. I find it somewhat rare that I have no idea what is going to be there unless it's a meaningless fight versus something like Drowners.
This. More often than not, you know what you're going to have to fight well enough in advance that you could adjust what skills you have enabled beforehand. There have been a couple of places I've gotten caught off guard, but I don't think manipulating my skills would have helped in those cases.
To me, the real problem with this system is that you don't get enough points for it to matter. I'm at level 17 or 18 now on the "Blood and broken bones" difficulty setting, and I have all the skills I've unlocked enabled. They should have either provided fewer slots, or more points, if they wanted to the player to have to adjust this frequently. As it is, it just seems kind of superfluous.