I personally don't think giving one of the signature aspects of rdm to other classes is a good thing. People complain about homogenisation all the time and you're asking for pretty much that.

I agree that it's silly to be unable to have a stack of phoenix downs but I disagree that they should be usable in combat. This would greatly change the raiding scene in a way that would reduce the difficulty of content. Yes rdm can res and heal without burning cds but they can't sustain that for very long and if they try to their dps suffers tremendously due to lack of mana. With phoenix downs there would be no downside in terms of class resource, and people could choose to res during boss downtime so they do not lose uptime on the boss.

However I would be okay with phoenix down being usable in combat in zones like Eureka content because death there can mean a loss of xp and lvls and it would have zero effect on the raid content.