If you have the budget for a GPS/cyclometer combo, then my thought would be "why wouldn't you want one?" Whether for road or mtn.

I can't speak for any of the non-GPS devices. I'm sure there are excellent ones. Me? I use a Garmin Edge 500 that quite old now and out of production. The things I get from it that I like are:
1) A record of my track
2) Heart rate, both real-time and logged
3) Calorie count, real-time and logged. But if you are going to wonder why one day you burned 200 less Calories than you did for the same ride another day, then there-in lie the devil. You'll be vexed forever.
4) Pretty much all the data you get from a non-GPS cyclometer.

I like Garmin's but some people have issues with them and they are on the pricier side. I've heard good things about Lezyne, they are much cheaper. If I was on the cheap, I'd get the Lezyne Super GPS.