It will raise other issues.
Issue for the game :
Currently premium time is 1 USD a day(cheaper if you buy a longer time),that is already very generous.Premium time is not even required to enjoy the game at all. I know a lot of players that don't buy premium time and are doing very well.Game economic was designed so that one player cannot play top tier ship only so that mid and low tier wouldn't be so empty that queue time is too long or cant even find a game.You are supposed to lose credit on average when you play high tier by design. Premium time with the credit boost is something that will remove this limitation, it is not meant to be justifiable for every player.You can already see it being a problem with so many players able to spam tier X ,tier 8 ships are suffering because they are put in tier X matchmaking most of the time.
Issue for individual players:
It will give players a sense of "gambling" premium time away each time they press Battle! button. More invested into each match ,more frustration when it is a bad match.Making the community more toxic. Here are some events i can see happening :
player saying"I am not playing today anymore ,not wasting my premium time"
rushing to die just because the game is not advantageous with ship lineup to save premium time.
player telling team to lose faster to not waste his/her premium time
Think of it as a subscription ,not buying a in-game recourse to spend.It's like subscribing to TV service.If it is charged by watch-time, not only is it a bad for the company since it is very hard to have an expectation of how much they make, wouldn't it also put a little stress on simply watching TV?