Can't go wrong with shield, it helps out greatly in the one area DM is lacking, aoe damage. Soul drain into shield charge is a thing of beauty.
Best guess in regards to soul drain not boosting toxic is because when the game first came out, there was no such thing as toxic damage. Things like the vahzilok vomit were classed as an unknown damage type if I recall, and it wasn't resistable. Soul drain being a power that also existed at the inception of the game naturally didn't boost a damage type that didn't exist. Subsequently there were no player powers at the time that did toxic damage so I guess this was just an oversight that never got corrected as the game evolved and it wasn't seen as a priority to correct.