Equity of that K5
Using pokerstove, if the raiser is raising with the top 20% of his hands and the other callers are playing the top 60% of their hands then the K5s has equity of about 13.5%, the callers 13.6% and the raiser 18.5%.
To make a call worthwhile based on pot equity alone the hero would need about 14.3%. But with the added value of the button, at that point you're actually in think about raise territory.
If the kicker was a little better, say a K8s, the hero's equity is about 13.9, the other callers 13.5. Still in the call territory.
What about A5s? It's worth a raise just based on equity. Against that 60%/20% lineup the A5s has 16% equity, the callers 13.2% and the raiser 17.8%. This is an example of a situation where you don't have to think you have the best hand for it to be worth a raise on value alone, you don't need to try to reduce the field to make it worth a raise, in fact you don't want to reduce the field.
How about pair? That's a hand Mason said he'd call with. A 66 is worth a raise here. It's in even better shape (just slighly) than the A5s.