I don't want to "artificially" limit my PCs. I want to realistically limit them.
A 2nd edition bard and a 3rd edition bard go into a bar, and sit next to a wizard. Both bards are 3rd level, and both of them *just* because 3rd level. The 3rd edition bard has already chosen his new 3rd level spells - thus, his "spells known" is already defined.
They go into the bar and sit next to the wizard. They discuss magic and stories, and have a great time. At the end of the night, the wizard says, "Hey, I just researched a couple of new spells. I'd be happy to share them with you guys, if you want to learn them." The 2nd edition bard says, "Great! I'd love to. I love picking up new spells wherever I go." Meanwhile, the 3rd edition bard says, "Gosh, I'd really ike to, but I've already chosen my new spells for this level. I can't learn any new spells until I gain a new level."
That's an artificial limitation. Another analogy would be the following:
A wizard creates a potion of blur. Then, a kobold enters his chamber, and he kills it - and then promptly gains a new level. He has _exactly_ the amount of XP he needs to gain this new level, and not 1 XP more. He tries to create yet another blur potion but finds that he cannot. Why? No more free XP to burn. That's an artificial limitation.
A realistic limitation would be - the wizard tries to create another potion of blur, but realises that he's used up all his Blink Dog saliva for his last potion. "Damnit! I'll need to get more blink dog saliva if I want to create another potion." That's a realistic limitation.