User blog comment:Waterwecna/Merlin: what happened to you?/@comment-5598897-20121104222540/@comment-5674726-20121105000733

if Arthur allowed Magic Morgana would shurly use that to her advantage more so than she already does.

Morgana isn't shy about using her magic now, when it's illegal. Why would she be? I can't imagine that she sees herself as having anything to lose at this point. Arthur can only execute her once, after all, so after her previous antics, using magic is unlikely to make things worse for her than they already are. I can't see that legalising it would make her any more or less inclined to practice magic.

However, as long as magic is illegal, other people with magic, who might be able to help Arthur defend Camelot against Morgana and any other magical threats, can't come forward to offer their help. Even Merlin's hands are tied to an extent, since he has to use his magic without letting anybody see that this is what he is doing.