User blog comment:FlamingShadow213/Right now's the perfect time. Don't ya think?/@comment-5674726-20121218064501

I'd say that it's vital for Merlin to tell Arthur about his magic now because, once the battle begins, it's not going to help if Merlin has to hide his magic from his side, or if Arthur sees him use it and is distracted from the task at hand, even for a moment.