I feel that adjusting the resist to -50 or more would still not make it that much more worth using a spellgem for. As a druid, I can already cast a 1.5k base dmg Wrath of the Skies spell (+ damage increment 5 = 1.9k), which isnt even relic and still get better dps. On summoned targets, I rarely go to the trouble to unmem one of my DD's and mem disposement just for the purpose of kiling the mob - Wrath of the Skies does the trick anyway.
Since Wrath of the Skies seems to never resist in the first place, I just don't feel like adding a -50 resist mod or even a -100 resist mod would make it worth it for most druids/mages.
As for Dissolvement of Shadows, I can't speak for necroes because I don't have a high level necromancer myself. I feel that if the recast delay was adjusted I *might* mem the spell when I'm in areas with large amounts of summoned mobs.
My spellbar typically looks like this:
1) CH
2) big heal
3) group heal
4) quick heal
5) another CH b/c i was too lazy to change my macros to put two ch's at top
6) DD
7) DD w/ alternate resist type
8) single root or rootcall
I'm not about to give up one of my DD's to mem disposement even with the -resist mod.
In summary, Disposement just isn't worth it, even with a resist mod. I don't miss that many casts to resists anyway, so a resist mod would not make it any more valuable.