I tried fermenting agave nectar (which is not the same as agave syrup) by itself. Let me tell you, it took a lot of yeast nutrients to get it to go. It was like fermenting honey to make mead, but much more difficult because it is almost completely fructose and brewing yeast is not designed to efficiently ferment pure fructose. Even honey has more nutrients and less fructose. I've been trying to get it to ferment down for two weeks.
However, if you were trying to make a braggot or add it to a beer to add sweetness, like a honey brown, it should be fine since you'll have plenty of nutrients and the maltose from the grains.
FYI -- agave nectar is the liquid pressed out of the agave root. It is filtered to clear and sterilize it. It tastes a lot like honey, and you usually find it in the store with honey. Agave syrup takes the nectar and refines it down to a neutral sweetener like corn syrup.