This is ABSOLUTELY necessary for support. I have a 55 gallon setup and the first tank I bought had a cracked center support and the gentleman at Petsmart convinced me that it was just there to hold the hoods and was not necessary. I took the tank home, glued the center support back together and started filling with water after it cured.
As soon as the tank was full, the brace cracked again, and let me tell you that I have never seen glass bend that much, but it was the scariest sight ever. Water immediately started leaking from the silicone sealed edges. I figure in about 30 seconds I would have had a floor full of 55g of water. Luckily I had a friend there to help me get it drained immediately.