Suppose you are playing longhand (more than 6 players) no limit and are in late position (CU or BU) and nobody has raised or called before you. In that case, I would raise any pair even if I only had 22. After all, I'm in position and probably have no more than 2 opponents. No Limit lives from aggressive play in position. I can take the pot with a continuation bet (C-bet) even if I miss the flop. In the middle position I would only call with small pairs and hope for three of a kind. In early position you can also fold small pairs because you can't play them as profitably from there. Even if you hit three of a kind, you have to be paid off by your opponents first. But that's much more difficult in an early position. In shorthand no limit I would always open raise with small pairs, no matter what position. In shorthand you have to play multiple hands. It is action-packed and lives even more from aggressiveness.
The question of raising with mid to low pairs may often be a factor of your stack depth and table dynamics. I would consider 6 and under as a low pairs and 10 and under a mid pairs and play them accordingly.