I find myself regretting completing the Purification Quest.

I was bored with the 'Dead Drop' Quests.

Now I have to work around the death of Lachance.

Am I wrong?

I believe it is worth completing the DB questline once to see what it entails. After that, I would urge any characters of mine interested in that questline to ignore the purification and simply maintain the status quo prior to the purification.

I find there to similarly be a 'sweet spot' where I 'freeze' the MG questline and advance it no further as well. My current character who belongs to the Mages Guild will never see

I agree.

Completely aside from the Purification itself (which really doesn't bother my assassin characters - that's what they get paid to do), I just don't much care for the rest of the quest line. The end is okay, and as Acadian says, it's worth seeing once, but the dead drop missions are just dull and really not worth bothering with, IMO.