Yeah I care about spell pen, but when my spell pen was 32 pre-expansion and it jumped to at least 38 without an ED in post expansion (no changes to gear) why do you care? Seriously, there is no reason you need to take more spell pen.
Also having lower cooldowns still is useful. Half the time I don't even realize that Wail or CoD are off timer, but I suppose it's because I use different spells too and don't just rely on them.
And 15% might seem low, but really it's not. It procs often if you use your necro spells (more if your a pm). Besides percents in this game are wonky anyway. Torc has a 1% chance to proc but it procs way more than 1%, same with the concord op.
If you want to have more fun with an ED, don't play the magister one. That's why you can take either Int or Cha in DI and twist in the +3 spell pen and dc's.