Yeah playing defence has value, but the defensive value a Marner type player brings is nowhere near what astrong defensive Dman or centre brings. Marner's problem is that so far, his offense dies in the playoffs. 100 point player during the season, 70 point player in the playoffs. The scary part is that the first 4 games of the playoffs he's still a 100 point player but after than, he's a 40 point player. On a team that always dies at the end of the 1st round, that stands out like a sore thumb and no, playing defence does not make up it and therefore, at least IMHO he isn't worth his cap hit in the playoffs.
Great regular season player though, he even helps kill penalties. I'll bet he's one of the worlds best shinny hockey payers too and even MacKinnon I think it was said that he sure looks good skating around in the summertime.