I think it would be useful because you'd be able to see your balance at the touch of a button, and then you could compare your actual Bank statement with your accounting bank statement just by looking rather than having to run time-wasting reports - after all, this is would your real Bank statement looks like, debit - credit - balance, side by side all on the one line. Why can't Sage do this?
See also:
Being able to sort the fund Activity screen (and have a running total)
Running total for bank transfers
Customer/Supplier running Balance to show on activity
Running balance on every activity screen
Product Activity - Running Total
Product Activity - Running Total (Duplicated when new ideas board set up)
RUNNING TOTAL
I guess that every [activity] screen has been hard coded for the specific screen, which means changing any/all of the above is a labour intensive task.
A solution (as proposed here (Move headers) would be to have a generic screen function for every [activity] screen that uses passed parameters to give the source data and columns, which would mean that any future change in them (eg adding a running balance column) could be easily and speedily done.