From the info I've been told, not really.
nah don't bother, get something worth while, downtime + active time r just the icing on the cake as far as stupidness goes 
On the whole I'd say no. If you think about it, do you really want a highly visible and aggro-happy nutcase following you from mob to mob? It's like trying to sneak up to someone with an errant toddler in tow.
Of course, that's not really an issue, since there's a massive 15 minute recharge time for them all, so you couldn't have them out all the time even if you wanted to.
That said, there's something to be said for having a Night Widow on your team. They're pretty effective in combat and have pool-power stealth, so that's a bonus. Shame I've gone Mu, really.
Are the summons in the ancilliary pools worth getting as a stalker?