Oh, the reason I went this route was twofold.
1. I thought it would be easier than painting the back of the plexi, which I think it probably was though I can't say for sure since I didn't attempt to paint the plexi, and with tinted plexi I don't think you'd be able to tell the difference.
2. This cabinet originally came with a dead 25" monitor that I pulled out and put a 21" computer monitor. I do hope someday to put a 25" or maybe even 27" monitor in this cabinet. If I painted the plexi I'd either have to replace the plexi, or carefully scrape paint off, when I got the larger monitor. This matte board method gives me the flexibility to change monitor size later if I want without having to replace the plexi.