Isaiah Thomas kind of typifies something I feel that people greatly underrate--how much defense matters and how much a star who can't play defense devalues his own offensive production. For example, I'm not entirely sure that the Cavaliers would have performed much better, if at all, in the 2015 Finals if they had had Irving and Love healthy. Yes, they lost firepower in losing those two, but they gained a ton more defense with Tristan Thompson and Matthew Dellavedova. In last year's Finals, after being blown out in two straight games with Love, Cleveland blew out the Warriors without Love (granted, it was at home, but going from road to home isn't worth 50+ points). They then lost at home in Game 4 with Love back.
It's very hard to quantify the effects of defense and a lot easier to quantify offensive production, but when an offensive star is awful at defense, it's probably unwise to consider them essential to the team.
(Also, before people use Curry as a counter-example, the stats don't bear out that he's a below-average defender and most experts that I've seen opine on the subject don't consider him a poor defender. My own observations of him are that he's an active and smart defender and his main weakness is simply size--not technique, effort or instincts.)
