The problem with calling with a lot of those medium to weak
hands OOP, is what do you do with middle or bottom pair? If you bet, then you may get called by draws or weaker made hands and then what do you do on the turn? If you check, you're just asking to be bluffed off the hand. If you bet, you're putting money in when you probably aren't ahead of his range. So really middle pair is just about as good as nothing when you're in the SB in these 3-way or more pots. And if it's 3-way, do you really have
odds to call for the 5 percenter flopped monster or the rare occasions when you do hit top pair on a low flop? Probably not. Now, if there are many players who have limped, I will probably limp with a lot of low connectors and suited hands and go for a monster, but I'm not putting anything in with one pair.
Oh, and regarding the scenario where just the button limps, why would you ever just limp in the SB? I'm raising there EVERY SINGLE TIME I'm playing the hand. The button limp is so weak that I think the fold
equity outweighs almost any hand you could see the flop with. I think there's an argument for limping big pairs there, but I'd probably raise them too in order to disguise my hand. I suppose if the limper is a tricky, aggressive player and it really looks like a trap, I might limp some drawing hands, but I think this is almost always better as a raise.