No matter if fierce assignment or non-fierce, unless a card says otherwise, you should be able to play an assignment effect if you can pay the costs and meet the requirement (for example spot). That's how the rules work, so
nothing will be changed in that regard. If you feel your opponent was trying to abuse the system and cause you to lose due to timeout, or loss of will to live, please post a game replay link and it will be investigated, and the abusing player will be kicked from the service.
If you still think rules do not allow that, please specify the reasons (quotes from rule book). For me, the only thing that can be questioned at this time is, if a player should be able to prevent an effect (per card text), if the effect is not playable. So in this particular case, should the FP player be allowed (and asked) to discard an unbound companion, if the
Southron Commander could not be assigned to the Ring-bearer (for example because it is not fierce). So far, it's not checking that, because I could not find anything in the rules that would answer the question.