I really don't like that spell/feat in general -- I think it is too expensive for the benefit; however, you make a good point. If EE Deathwyrm is your focus it could make sense, as many of your cheap nukes (necrotic ray) are not that useful against red/purple named undead, and it gives you a decent (if expensive) direct damage option.
I would still hesitate to drop my DC's down to fit it in(wining the lottery and pulling a +6 int tome makes it easy though), although depending on the content you are running, it could be a nice option to add versatility.
If you take Ruin, I would also drop Echo's of the Magister and twist in Sense weakness instead (no need to have an odd Int).
I haven't really looked at Ruin in a while -- running through a bunch of ETR's atm, might take that next life and see how it works out.