I think one reason that adding “tension” to a thesis — or, in other words, making a thesis sufficiently nuanced — is important is simply because it makes the thesis more interesting right off the bat. It supplies an implicit reason why the thesis is important. And, like I said, it enables the thesis to draw the reader in more quickly.
Dodie commented at the staff meeting yesterday, that when she heard our examples of theses with tension (she had each of us come up with our own — though I opted out because I’d already come up with several that morning, with Chanel), she realized that just starting out with an “although” or “whereas” statement (or some such — whatever works) made the thesis more interesting to her. I think she said one reason might’ve been because when she heard the “although” statement, she wanted to jump in a give her own response. But it could also be because it makes the reader curious about what the “other” side is — the new side…. the”twist” on the old truth, so to speak.