The only argument I've seen about leaving batteries in the tank is that the Tamiya on-off switch does not isolate the battery, and they have a tendency to lurch forward once the battery discharges down to a certain level - I remember there were a few post about tanks leaping off shelves on here back in the day. I think everybody else (i.e. HL etc) uses a proper on-off switch that isolates the battery completely.
The other thing to watch for is the screening on the cables starting to split if they are being stuffed into a standard HL battery compartment - a few years ago I started to see this happening, and since then I've taken to cutting openings in the ends of the compartments in my tanks to allow the cables to poke thru and not be crushed between that and the end of the battery case.