Attaching a bayonet doesn't make shots impossible, but it can affect point of impact. The anecdotal answer in Garand shooting communities seems to vary anywhere between a 1 to 8 inch vertical shift at 100 yards, and an opening up of group sizes.
The vertical shift can be compensated for if you know what to expect on a given rifle (I'm going to assume a lot of little variables adding up between different rifles to account for different anecdotal differences), since pretty much everyone reports that a given rifle with a given bayonet is going to have a predictable shift.
There was almost certainly value in training to shoot with the bayonet attached, since attaching a bayonet was a plausible combat reality. Just because a bayonet might have degraded accuracy to suboptimal levels didn't turn the Garand into a non-functional firearm.
