I'm not sure I see it as a disservice to the characters but rather a messy chapter with future consequences in an ongoing story. It is a morally questionable choice that people make in real life. I like seeing them make morally questionable choices. I love that they kissed like crazy at the end. I also love how it's going to come back and bite them both in the ass.
I'm quoting myself from another thread here:
Hollywood is, generally speaking, bereft of morals. In that vein, it is completely expected that they would morally corrupt two characters this way, because most of the movers and shakers in Hollywood don't even see it as wrong.
Spock is an honorable guy. (Or at least he used to be.) I very much wish they could have gone down the same road that Herman Wouk took with his character Pug Henry in
War and Remembrance, when he was torn between his unfaithful wife and the character of Pamela Tudsbury. When the latter suggested that they just 'shack up' and take what joy they could from their lives, Pug discarded that idea outright, claiming that if he was in love enough with Pamela to do that, then he was in love enough with her to divorce his wife and marry Pamela. Later in the story, that is what happened, and Pug remained physically (if not emotionally) faithful to his estranged wife until it was officially and irredeemably over.
In this case, the characters of Spock and Chapel both would have been better served if, when T'Pring suggested a break, that Spock simply made a clean, permanent break and then gone to Chapel with a clean moral slate, heart, and conscience. Sure, the story is more dramatic this way, but at the expense of our characters' honor, as it were.