Why though? Basically, what you're saying is that there's no point to play a holy mage in a tournament if you're taking away one of its strengths. If you see/know you're playing a Priest/Priestess and know this is an option, you've got to prepare for it. Jinx, Reverse Magic, Nullify could all be played ahead of time or don't let the Holy mage survive long enough for it to go to time.
Restore/Heal/Sunfire Amulet...these are are all aspects of the game that have to be dealt with and to me there's no way this leads to anything overpowered because they can't be played in a massive chain. At most, a holy mage can gain 19 life on avg at the very end of the game IF and only IF you're out of range to stop both actions. There's no way they can cast both of those spells without you having a chance in between to stop it via Quickcast.