Lithium Polymer 12V starter battery equivalent, are now available in whatever MAH you might desire, so apart from the difference in price between these & glass mat lead acid batteries, the balancing problem has been overcome I believe.
As to why bother there is the small matter that unlike lead acid batteries you can use nearly all the capacity of lithium polymer battteries, deep cycle lead acid batteries, won't wear being regulary dicharged, below 1/2 capacity without serious life degredation. And there is the obvious weight factor to be taken into consideration . Lithium polymer batteries are featherweight in comparison to lead acid batteries. We just have to wait for price parity & below & everyone is a winner...