It makes me sad that there are hardly any supporters of this pairing here.
Aside how incredibly CUTE it is often depicted in fanarts... I also think it that there could be a much darker and interesting aspect to it as well. Itsuki and Mikuru don't really trust each other. They're not supposed to, what with their groups having different agendas and all. And yet both being in the SOS brigade, they are forced into situations where they have to depend on each other.
Not to mention, with Itsuki's mysteriousness and Mikuru's moe charm, they could even try to manipulate each other in order to find out more about their organizations.
I have a feeling that if they did somehow get together in an odd, parallel world and their superiors ever found out - It would be BIG trouble, especially in Mikuru's case. So in that sense, ItsukixMikuru is a forbidden love.
But for the most part - CUTENESS. <3
(Pic source: http://100.tobiiro.jp )