How useful is a 50 damage rune on enemies that have 1200-1400 health? and on master difficulty it only does 25 points of damage. I know you can dual cast them but really they are pretty much as useless as t-its on a nun.
For me, the initial motivation to use runes was the fact that it is cheap and very good for gaining experience. A Draugr Deathlord has maybe 600 health. And in Master, I would have to hit it 24 times (600/25 = 24). A melee deathlord has strong attacks but is sluggish, so it's very easy to outmaneuver him and never get hit if the room is big enough. There are times when I feel that the situation is rather dangerous for my character and feel the need to dispatch the opponent quickly, in those cases I guess I would use incinerate, or firebolt with impact if my magicka reserve is not high enough. Other times, it is quite safe to kill an opponent slowly, especially if it's giving me extra XP points.
Destruction spells have some advantages over melee or archery. Destruction has longer effective range than melee, and they are more mobile than archery. At this point, my personal preference for weapon damage in late game is around 50 damage sword and 100 damage bow on adept, and 100 damage sword and 200 damage bow on master. For me, Destruction magic falls within "fun" range on adept and on master. Some people like to play with 600 damage swords. I think it's rather boring to play it that way, and I wouldn't want to play like that, but I have no problem if other people want to play like that. Those that prefer playing with 600 damage swords would probably find Destruction rather weak.