How exactly drain work ?

[Eressil3]

I never took drain in MP because I love magic.

But I'm starting to consider it.

1- is there a reason to not take Drain 2 over Drain 1 ?
2- does it have negative side effect like reducing for instance the number of creatures summoned/reanimated ?
3- does the enemy gain MR under your dominion ?
4- is there something undocumented about drain ?
5- why do/don't you like drain ?