Based on what I have been reading here, the tanks with rims don't have well finished top edges. So, when people remove the rim they find rough, uneven top edges of the glass. Unless the rim is made of metal, such as stainless steel, and has a wide flange at the top, it isn't contributing much if anything to the strength of the tank. Glass is a very strong material, brittle, but strong. I suspect the rimless tanks cost more because of the extra care and effort it takes to get a good finish and even edges at the top, but not because thicker glass is used.