I think his point is that you'll still have fun playing the main story, but it'll take far less time away from you, and still leave you with the satisfaction of completion.
If you go for finishing side-quests, you may not feel satisfied until you've finished them all. That can take considerably longer and be considerably less worth-while than the main game. (e.g. saving princess and defeating nemesis vs. collect all 500 random blue hidden items. Both fun, one takes far less time, and is more "worthwhile".)
Fair point, although it depends on the game; I enjoyed the sidequests in Oblivion more than the main quest and would submit that most players would be really missing out if they didn't finish them.
Of course, that's quite a different thing to timesinks like "find all 400 hidden widgets" which can get pretty tedious...
Assassin's Creed II has some good examples of worthwhile and non-worthwhile side quests. I cannot imagine completing the game to one hundred percent, but half of that is activity which reveals more about the story (tombs, "truth" encodings), while the other half is just numbers (collecting feathers, weapons, and paintings, although I admit to having taken interest in the last one having been an art history minor).
If you go for finishing side-quests, you may not feel satisfied until you've finished them all. That can take considerably longer and be considerably less worth-while than the main game. (e.g. saving princess and defeating nemesis vs. collect all 500 random blue hidden items. Both fun, one takes far less time, and is more "worthwhile".)