Realize this is a very old post, but since I spent a LOT of time on the FW side of things way back in the day, I thought I'd chime in.
First retailer I worked in ran UG filters on a blower, with heaters in each. Worked "so so".
Second retailer I worked at had all the fish systems on central filtration. This worked better. Regarding disease, all systems had UV sterilizers on them. Just as important on a FW central filtration system with loads of fish coming in and out. If a UV went out or went without a change though, forget it, disease spread (just like all the other comments). There was some "stability" and "ease" of working with the systems this way. Tanks did have gravel on the bottom, which was cleaned regularly as part of the water changes.
Third retailer I worked with had the old DAS retail systems with shared internal power filters. Despite all the headaches of maintenance (largely pumps going out) they actually worked pretty well.
When I went to the wholesale / hatchery side of things to do the Cichlids, it was a return to individual tanks all running with sponge filters off blowers. Some of the really large tanks or tanks with large fish would have supplemental power filtration. It took half a day to do 50% water changes weekly, but man did stuff GROW and do well. Still, it was a lot of work, and in such a large scale setup, it's arguable that central filtration would've made life much easier. But I think this was a much more economical route to go.
I'm planning to keep a few FW tanks again in the near future if I can, and while I could probably put them on a central system, I'll probably just keep them separate.