That is a hard thing to answer because it depends on you and what you consider moderate single track. I mean I run 28mm road tires 95% of the time on my cross bike but when it is dry enough and I feel like it I'll hit a couple miles of single track on one of my normal road loops just to mix things up with those 28mm tires and it is ok, just have to slow down. I actually think one of my fastest times on that particular trail is with my cross bike on 28mm tires, the dirt was just perfect for it that day though and it is a fairly smooth trail. Now when we know we are going off roading I run 39mm tires and I can ride all my normal mtn bike trails on them, maybe not well and there are some parts when climbing steep rooty sections with cross tires is really hard if not impossible but for the most part I can ride everything. If I were going to do that on the regular 2.1s sound pretty good, my rigid mtn bike runs 2.4 front and I think 2.2 rear with flexy carbon bars and ESI super chunky grips and I still woudn't mind a bit more cushion. My main bike is a 140mm FS trail bike though and I'm spoiled.
I guess having the ability to run wider is good. Means you don't have too but you can.