to my knowledge the plastic on the bottom is the transfer medium of pressure and weight of the tank onto a stand, it also adds some protection to the glass and gives strength to the corners. the plastic on the top of the tank, if a center brace is present stops the glass from being pushed outward (bowed) by high water pressure, usually only needed on tanks 40+ gallons large.
if you have the means you can always substutate the plastic framing with wood (both top and bottom) and/or at the top of the tank, you just use a peice of glass siliconed to the sides to act as a center brace.
or if you can build the tank to manufactured aquarium dimensions, then you can special order the plastic frames from a manufacture or
LFS. i had to replace the top brace on a 55gallon tank a long time back, i went to my local
LFS special ordered it for 16 bucks.
I do not want to build a plywood tank simply because I do not have the time, room and equipment to dedicate to such a project. I local auto glass company said they could build a rectangular glass tank to my specifications and this is where my number one main concern comes in: do 'all glass' aquariums NEED the plastic/steele/wood frame work around the top of and bottom edges of the tank? If you don't know what I mean think about the typical 20 gallon aquarium available at WalMart and the black plastic peices on the top and bottom. If the frame didn't add structural integrity to the aquarium then I wouldn't be concerned however that is just not the case. Or is it? Is a silicone sealed 'all glass' aquarium strong enough in it self to hold that much water without bursting at the seams? Any feedback would be greatly appreciated. 
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