Harv,

I think the rule of thumb you are quoting applies to situations in which you have already invested a full small bet. A +EV situation is defined as one where you will, on average, expect to earn more postflop than your original investment (I'm pretty sure this is the definition provided in SSHE, although someone should check this). Calling for half a bet is different than calling for a full bet, even though the current pot odds are the same. This is because the bet sizes postflop become relatively smaller to your investment.

There was a hand recently on here involving 55 on the button, that was faced with 3 bets cold preflop after 3 cold callers, basically giving the poster 5:1 immediate odds on his call. Typically, this is where most players would cut off calling for set value. The odds for flopping a set are about 7.5:1, meaning that the OP would have to make up an additional 2.5 small bets postflop to make up for this. This would be an easy call if it were for a single bet, as 1.25BB is not that hard to make up postflop against 4 loose players. But the situation was for THREE bets. This means the OP would have to make up 3.75BB postflop to make up for this. Although the immediate odds are the same, the implied odds are significantly different.

My point is that the implied odds for calling one small bet and then an additional small bet are identical; however, to call one small bet and pay twice that when it comes back to you is different. If calling for half a small bet in the SB is very marginal, then folding to a raise is mandatory.

EDIT: to include link.