Not really. The prefix check is helpful, but when would you use it other than in input scripts as a way to determine if you want to use your script on it or leave it as-is so the command works? If there is some other need for it, then I agree that it would be worth adding that. But I can't really think of any other time you're using that.
And if you're using the prefix in a check to see if you should run your script or leave it as-is, then you should also be checking for $ctrlenter and $inpaste. Rather than having to check all of them in every input event, an $iscmd check (aka $command check) would make a lot more sense.
So from my point of view, $iscmd makes the most sense. If there's a reason why you'd only want the command character, then adding a prefix identifier as well is okay and it would make sense if you did that at the same time anyhow. But having a prefix check without the command check just doesn't make any sense.
EDIT: Btw, I was thinking of $iscmd just because $command was mentioned. I'm not sure it's the best name for it since we're also checking $ctrlenter and $inpaste. But I'm not sure what else we'd name it. $isspecialinput is way too long and hard to read.
Maybe $safeinput or $protectedinput, but even those just aren't very good either. Maybe $iscmd would be fine and just clearly explain everything it checks in the help file.