Hm. I don't know what decklists you're looking, but Thoughtseize is the lynchpin of Esper Control. The deck absolutely needs it, unless you're rolling with a budget list. Check out this site for what's being used in the major tourneys. http://www.mtgtop8.com/archetype?a=197&meta=52&f=ST
EDIT: Of course, the Grand Prix deck didn't run them...so...uh. Nah. You still want 'em. lol
I'd say the lynchpin is still the Azorius trio of Rev, Sphere, and Verdict. Thoughtseize is certainly a good reason to put black into the deck at all (along with the various removal options) but there are occasions and variations in the meta where you probably don't want 4 of them. Sometimes you're just facing a lot of decks where it isn't as good as it could be because they have such a high density of somewhat interchangeable threats.
Edit: Not that you were saying you need them as a four of.