What kind of backing should I use for a glass mosaic backsplash?
Here are some good points against using plywood, the source is talking about tiling a floor but most of the points are valid for walls as well.

In my opinion it's better to go the "overkill" route, then have to redo the job in the future because something didn't turn out right.

The cost difference between plywood and cement board is negligible, so your not going to save any money using plywood (unless you already have it).

Do the job right and use cement board, when you don't have any problems and you can enjoy the tile for years to come you will never regret the decision.