I like having the option of being able to switch between characters so that I can see at a glance who is doing what. I know that I'm not a supporter...
yet, but it's still a really neat incentive for me to become one!
As for the concerns of who is the author behind, as it had been stated, just change the title to that of the 'host' or 'main' user name. So that, at least, is addressed. Same for if someone has to resign their position within a faction or company and reassigned to others.
As for the 'what if someone forgets to change their sub account' issue: This is a forum. Most of the plots are set to where you post within 3-5 days from the time the GM posts. Unless you are just not giving yourself time to really think of what it is your character is or is not doing, you should have time to switch between characters easily. Otherwise, just post in your 'main' account as each if you feel there is no time or wait until you do have the time to switch.
But this is just my own little humble opinion. Take it for what it's worth.
Addition:
In regards to whether or not this is good for site supporters, let's think of it this way: GMs and FMs we know are responsible enough to not abuse this. For the most part, those that are 'regular' site supporters...meaning those who give their support each month...are either GMs, FMs or former ones, so should also be just as responsible. Those few who are not can easily be monitored and, if need be, the option taken away. This is a privilege, not a right, so making that clear at the start would also be a good idea.