The main problem with rule #2, as I see it, is that it can be carried to certain illogical extremes. If meteor swarm were to be made a 1st-level wizard spell, it's true that NPCs would gain just as easy access to it as PCs, but would this be a good idea?
I nerf things IMC all the time. However, I have some basic rules about nerfing a spell or item that is in play:
1. No rules changes can be enforced until the beginning of the next game session.
2. If the spell or item is in possession of a PC, the player has the right to choose a spell of equal level, or an item of equal gp value, in place of the "nerfed" spell or item. In addition, each player gets an XP bonus of 50 x level, in order to compensate them for being inconvenienced by my lack of foresight.
If I ever nerf something that's not a spell or item (a PrC ability or somesuch), I'll probably have to figure out a different means of compensating players for the inconvenience. We'll see.
I do have to say that DMs who nerf spells, items, or abilities just because they're used effectively by the PCs are just taking the PCs' success a bit personally. The only real reason to nerf a spell or item is if that spell or item is consistently superior to all other comparable spells or items, or if the spell or item just doesn't work with the rules as written. (For example, there's no way to deal with unerrata'd speed armor or spelldancing infinite recursion loops using the rules.)