About the only possible downside I can think of, is that it helps those in the 'know' sometimes to skip queues and bag a good seat - especially at places like Euston, where unbooked table seats, forward facing, with a window alongside, are very few - and often only in the unbooked carriage. If you don't know which platform it's boarding from, you haven't got a chance. TOCs may view passengers boarding early as an inconvenience or getting in the way of train preparation. Although I can't see why - I've never had any negative comments from train managers or train preparation staff. I'm always courteous to staff who clean and tidy-away the less left behind on our trains and stations, and often thank them for their efforts - it's so easy to take what these people (often on low wages) do for granted. And I often find other people boarding ahead of time - particularly regulars taking a gamble that their train will be on the platform it usually leaves from.
There's a solution available to TOCs that don't want people boarding early - don't open the gate line. I'm happy to be the first in queue at the gateline - that way I still get to be one of the first on board, bagging a decent seat.