Yes, you can tap-load a musket for a couple of shots, IF, as was typical, the ball is a loose fit in the bore. The tricky thing is that as the bore fouls ( and black powder creates a good bit of fouling) the ball will be tighter and tighter, and the fouling could also accumulate a little heavier at the breech end- meaning, you might think the ball is still a loose fit, but it only drops halfway before it stops. Now, if you were calm and everything was quiet, you might be able to tell if the ball had gone all the way down, it might make a satisfying "klonk" when it hit the breech end. But this would not be possible all the time, especially in a battle.

If the ball doesn't go all the way down, upon firing the barrel may swell up at the point just behind the ball ( more common) or the gun might actually blow up. Either would put the gun out of action.

There's also the problem of a loose ball simply rolling back out of the barrel, if the gun isn't kept inclined, or upright. Or, if it rolls halfway out, the barrel could swell on firing.

The loading drill with wad and ramrod would ensure that the solider would continue to have an effective musket.