You raise a good point about one battery pulling down the whole bank. It is a good idea to use a multimeter once in a while to see if all the batteries in a bank are the same voltage. If one is bad, replacing it is a good idea even though the new battery will now die at the same time as the rest of the batteries in the bank.
I've never rotated batteries because, in my case, moving an entire battery bank increases the chances of my doing something stupid like dropping a wrench onto a live battery or breaking a battery case while torquing down terminal bolts. If you ever have reason to take out a battery bank anyway, rotating would be a good idea.
Instead of rotating batteries, it might be a better to put the time into using a more advanced method of making parallel connections. Bus bars on positive and negative are still in their infancy, but they solve problems of jump wire resistance. I think we'll see more of them. Here are a couple of examples:
The trick with bus bars is that the cables from the batteries to the bus bars have to be identical length. You also need a fuse on the positive side of each paralleled battery or battery set. The fuses look like this:
If you want to parallel four batteries (or four sets of two batteries) there is a way to do it that keeps all of your cable resistance the same for the whole battery bank. It is method 4 in the attached link: http://www.smartgauge.co.uk/batt_con.html
L-16 pricing suffers from the "Walmart problem." If you are comparing apples to apples, L-16s and GC2 golf cart batteries from the same vendor will be the same or nearly so on a cost per amp hour basis. I last compared Fulll River brand AGMs and they were within a penny per amp hour of each other. Problem is, you can't stop by Costco or Walmart and pick up a large L-16 industrial battery like you can a golf cart battery. Try as they might, the brick and mortar stores that sell batteries like L-16s can't compete with the Walmarts of the world. The golf cart batteries from Costco or Walmart still work out cheaper per amp hour than where you have to go for an L-16.
There is weight. Golf cart batteries at about 65 pounds aren't that bad. I can lift a 115 pound L-16 by myself, but would lots rather do it with a friend holding the other side.