I started with an A200 about 8 years ago, and my collection grew to a height of A200, A700, 2x A900 with a total of I think 8 batteries - 7 with Sony branding, 1 without (I am not sure if all 7 were genuine Sony).
I now have 'just' the 2 x A900, and 6 batteries (2 in VG + spare for each), and have no idea which of them these are (other than 1 being the unbranded one, as I sold both the A200 & A700 with apparently genuine batteries) - and have not noticed any real difference in capacities.
If I shoot a lot, sometimes it switches to the second battery in the grip.
I take the spare because it takes very little space, and the long life means I sometimes forget to check the levels of the batteries in the grip before setting out, so even if one is empty and the other partly drained, I still have a days shooting life
Keeping the battery constant through the Alpha range is one of the best moves from Sony - I have friends who use 'other brands', and a new battery for each new model is one of their biggest gripes.