Anyone else have an opinion on this?

The later position the raiser, the less inclined you should be to call.

The better the opponent, the less inclined you should be to call.

The thought process is:

If we are calling with pocket pairs with the intention of check-folding when we don't hit our set or have an overpair to the board then we cannot profitably call when against someone who is raising a very wide range of hands as we will rarely get much of their stack the times we DO hit a set. Ie, if someone is raising Q9s OTB and we call, are we getting their stack if we hit a set and they miss? No. If we hit and they hit top pair? No. Basically, we need to both hit a set AND they need to have a strong enough had that they are willing to felt with that is not stronger than our hand for calling out of the blinds to be +EV.

That is without taking into account the possibility of winning at showdown unimproved or the times that you are able to move your opponent off his hand.

Taking what Xaston said - The later position the raiser, the greater their range is and therefore we will, on average, get less money out of them the times we do flop a set.

The better the opponent, the less likely you will be to make it to showdown with your hand and have it be good, as well as being less likely to bluff your opponent off the best hand (ie make him make a mistake).

The fact that you are out of position is pretty significant as well.

I hope this helps.