As it is an offensive ability, it does tend to lend itself better to DPS jobs, that's true. Though there are a few things to note:

1.) Cross-classed Mercy Stroke has a 90s recast time, whereas with the trait from MRD it has a 40s recast. In the time that it will take a MNK/DRG's Mercy Stroke to be back up for a second go, the WAR will have already done 2 and 10s into building a 3rd.
2.) While it does only become active at 1/5th of the fight, this doesn't take into account hard hitting adds. Getting those killed ASAP is pertinent to maintaining up time on the boss. So the quicker the adds die, the quicker the DPS and yourself can get back to the boss.
3.) Typically a boss nearing the end of it's life is when it's at its most dangerous. Bursting it down at this point is a good idea and Mercy Stroke really shines at this point of the fight. If you could just use it whenever it'd need to be retooled or it would be an incredibly large DPS increase over the entire fight at 0 cost (similar to Heavy Swing, but imagine a free 150 pot attack every 15s and a free 200 potency attack every 40s. That adds up fairly quickly).

I think adding an actually useful buff to the ability is fine, but I don't think we'd get a very useful one if they had to retool it to be used the entire fight. 20% of your max HP is a pretty large heal and I'd rather have that and the limitations on the damage, than a much smaller heal every 40s throughout the fight. It would at least give us a reason to maybe hang on to Mercy Stroke for that extra heal rather than using it every time it's up.