And, as for this particular episode - Mirror Tuvok. Really? Was it really necessary to have him appear here? I mean come on, VOY had only aired ten episodes at this point. Most meaningless character crossover ever!
Yes, I have to agree. It would have made more of an impact to see Mirror Tuvok later, when Tuvok Prime was a more established character. And we'd already seen a "rebel" version of Tuvok in the pilot when he was undercover with the Maquis (however briefly). Plus, if I recall, Mirror Tuvok was barely a presence in this episode anyway. Seriously, it's like Tim Russ just decided he wanted a few extra bucks in his paycheck that week and wandered onto the DS9 set without anyone noticing until filming was done, and then they decided it was easier to keep his scenes in than to try and cut them.
As for the MU in general, I agree with the "diminishing returns" theory someone posted upthread. But while Intendant Kira is my favorite part of the MU, I think my favorite moment is when Mirror-non-hologram-Vic Fontaine is killed. I seriously think that was a shout-out to all the people who hated Vic (like me!). They couldn't get rid of him in the Prime Universe, but at least we got to watch him die here. I actually cheered when that happened!