I don't think that's necessarily the case. You might only have 4 second level spell slots, but you'd also have 4 first level spell slots, third level spell slots, etc, etc, etc.
There are only so many "rounds" of action in a given day (I use "rounds" here in a subjective sense to refer to instances where a spell might be useful). As the number of spell slots exceeds the number of rounds (which happens quickly when combat happens only once or twice per day and lasts only a round or two), the only REAL balancing factor is the potential for the wizard's player to guess wrong when he prepares his spells. And as the number of spells increases, the likelihood of guessing wrong goes down because you can cover more bases.
Another thing is that in pre-3e, wizards have a limited number of spells known per level. Once a wizard has learned a spell, he can't unlearn it and it counts towards his limit.
Pre-3e, the wizard doesn't want to learn knock coz he'd much rather be able to learn something else.