We've used it. Your point of colour is correct. I've top dressed alpine troughs, etc. with aquarium gravel. Of course, it's more expensive than most grits and too pricey for more than limited amounts used as a grit or decoration.

I agree- cost is the only real consideration- it's pretty expensive in quantity.

Just to add:

Other items I've used to top-dress a trough or alpine bed:

-marbles of various types
-shells
-agate/quartz chips
-pine cones
-various types of pathway stones. Most places that sell decorative gravels will sell a couple the bucket fulls for 'almost nothing'