The United Front tactic is useful when (as stated above) you can get something done for the working class by working with others. HOWEVER, in any United Front (and this is where most of them fail), the communist must ALWAYS reserve the right to criticize it's front partners openly and strongly, when they fall short of the mark. IOW, the Front is NOT more important than the communist programme.
That's why the front tactic works best only on single ISSUES, and not on things like electoral politics. If a communist expects to get closer to socialism through electoral work, he or she is bound to be disappointed. Kucinich and Sanders ultimately will come down on the side of the bourgeoisie BECAUSE they are social dems and not communists. This doesn't mean we don't vote or that we boycott elections other than in the cases when there is no choice other than a fascist. It just means that we vote to show that even the MOST left of the bourgeoisie ultimately will NOT get anything done regarding the system itself. At most it will be stopgaps against capitalist exploitation. But because we DID vote and not obstruct the "left" bourgeoisie AND STILL NOTHING CHANGED, we can say that "We tried it your way and nothing happened. Now let's try it mine."
To me, that's the whole purpose of electoral politics for a revolutionary socialist/communist. To show that the electoral system will ultimately fail the working class. And maybe to get a few short term gains that benefit the working class.