Absolutely, without a doubt, there IS an advantage to having Bar-Ends on a flat bar bike such as a hybrid or mountain bike!
1. Climbing tough hills out of the saddle is made much easier w/ bar-ends than without them. Bar-ends allow for more leverage.
2. They do give you more options for hand positions (can also help to relieve fatigue in arms, shoulders, wrists, back), especially if you use the kind that I use (Bontrager) that have a distinct bend/angle to them.
3. They can have you more aerodynamic by stretching you out more, if need be (good for long flat sections of trail/road, or for riding into a headwind).
Hope this is helpful. Be safe!