Attacking 2v1 against a location which defends with a +1 bonus (on d6) and which offers a bonus unit to the owner...or leaving 3 units adjacent which the opponent has to be concerned with. A take of the stronghold, of course, garners the conquering player the bonus as well as 2 units to defend the stronghold against future attackers.

The reason I ask is that in a current game, my opponent left 3 units on my doorstep and I was kinda glad he didn't try for the stronghold. Did he make the percentage play?

Is the 3-stack also protecting a bonus?

No, just regular territory.

According to my dice calculator (http://prestopnik.com/wargear/), 2 attacking 1, with 6v7 dice is almost exactly break even odds.

If it's a 1v1 game, I'd say it's definitely worth attacking.  With more players the global politics probably dominate.