Sadly there is too many problems with this.
Say you wanted to add your own alts to a FC but don't know who to ask, just make a Alt, Invite it from a Free Account, then just add your main account alts.
Or make a bunch of free accounts, and have them be in a FC By Themselves. take over buildings run by Free Trial Players once the Paying Account is gone.
There is merit, but there is also the risk of opening new floodgates for issues. you'd need to really comb through all potential drawbacks. Which they might not even want to open up.
