Hi Paul - the locking mechanisms seem to be hot right now, and while are especially good for frag shows, etc, I'm seeing quite a few using in their permanent setups for whatever reason (prevent frag movement by urchins, snails, etc I guess). I'm also seeing quite a few magnetic with colored acrylic sandwiched between to pieces of black acrylic designed to look fancy in displays.
When talking permanent frag tanks (racks on legs):
The obvious con of many of the commercial options is the price point. It's hard to justify acrylic when egg-crate is readily available and significantly cheaper. Commercial options generally look nicer though, maybe more professional, and are less work to put together. Other than that, I simply don't see much advantage over egg-crate. The one exception being people that do frag shows. They might prefer a commercial option in the frag tank for easy transport. I recently got some racks from building an obsession for this purpose (locking, fit perfectly in a coleman cooler, and stack with plastic dowels) and they are admittedly awesome for transporting to shows. i leave them in my frag system in the meantime so they are ready to go for the next show.
When talking display tanks (racks on magnets or suction cups):
Because these are generally considerably smaller, a simple commercial rack is much more affordable and ready to install. Egg-crate here is considerably more difficult to deploy as mounting it will prove challenging to most. I would never use either with suction cups. Far too unreliable to be stacking high dollar frags on, and too much risk they end up dumped in the tank.
For me, the notion of affordable "board" that I can simply cut to fit my frag system would be awesome. A few threaded holes spaced appropriately on the bottom to install legs of various lengths into would be great too. Ideally the vendor would offer these as well. Simple plastic dowels threaded on one end would do the trick. I suppose a 12" one as a standard could easily be cut to whatever length the user desires. I've seen quite a few different styles and methods here (e.g., short legs in front, long legs in back to tilt the rack toward the viewer). Hell, you could even put some holes on the sides of your board, and sell encapsulated magnets with threaded male sections to screw into them. People could cut their own magnetic display racks if they desired, or make a couple out of whatever scrap maybe left over from outfitting their permanent frag setup.
In my dreams, my frag racks look clean, are dirt cheap, resistant to algae growth, and are loaded with coral
Hope this helps some. Looking forward to seeing what you come up with.