The tank ran as a reef for 4 years with each corner of the frame cracked & separated. Ive recently taken the top frame off and the plastic is pretty flimsy so its hard to believe its there for structural support. Seems like it would bend with the tank if it bowed. The tank is sitting in my basement waiting for me to decide what to do with it.
Top and bottom trim has several purposes:
1. It does provide a rigid frame that helps prevent top and bottom corner seam separation. Note the "helps prevent" part. Tank seams, at whatever size tank, can and do fail at various tank ages with the trim intact. There are no airtight guarantees.
2. At taller tank heights, a center brace helps prevent center bowing, thereby preventing further stress on corner seams. They also facilitate the installation of glass tops and covers that now don't need to be 3 feet long in some cases.
3. The added integrity that one piece trim provides, lets manufacturers use thinner glass. Think of all the cost saving avenues here, there's a bunch.
4. It covers the most dangerous part of any pane of glass and eliminates the need to seam (polish) the exposed edges. More cost savings.
There's more, but all I'm trying to emphasize is that you shouldn't plan on the rimless route simply because the tank held water with trim that was broken and cracked at the corners. All I'm advocating here is that if it were my project, I would build in some peace of mind factors. If you're OK with tearing it down and reassembling it with Momentive RTV, understand that I'm absolutely not writing all of this to tell you you're wrong or making a mistake. There's lots of ways to achieve things like this.