Yes, there are those who use marine grade plywood for non viewing walls. They don't have to look bad, depends on your tatse really. I routinely paint the back and sometimes the sides of my tanks black, in that case, what the wall is made of is totally imaterial!
It is more expensive then regular plywood, yes, but cheaper then a piece of glass that will do the same job. You don't say where you are so it is difficult to comment further.
I think Fozzy is building parts of his giant setup with plywood.