Good idea, especially if you've already got a cargo carrier, or have use for one. Like all DIY projects, it depends on the skill of the operator, though. I think the success of this setup will depend on how well you secure the rear wheel.

Biggest pro is the cost. Some of those cargo platforms are really cheap, and so are the fork mounts. Heck, there's someone on my CL who's selling a pair of them, plus a wheel carrier for $15. You could easily knock this together for less than $100, maybe less than $50 if you get lucky with used stuff.

Other than cost, I don't see any advantage over a dedicated bike rack. I have fork-mount trays on my roof rack, and they're great, esp. for long hauls, like taking the bikes on vacation, where I may be in and out of the car several times before I need to get the bikes down, so they're out of the way.
Biggest drawback to a fork-mount is that you have to take your bike apart and re-assemble as part of the load-unload process.
I added a wheel-tray style hitch-mount carrier for ease-of use. Basically you undo one wing-nut and the bike is ready to roll. Makes for quick loading too, if you need to make a fast getaway.