My son is going crazy waiting for updates on the Zerg. They're his favorite, too. I just tell him, first of all, this is an excellent opportunity to practice having patience. We got a little of each race in that first splash, and we've seen Terran and Protoss now, so Zerg will obviously be next. I think Blizz knows that Zerg are a lot of people's favorite race (for whatever reason - I personally hate the little buggers) and I'm sure them holding out on Zerg details until last is not an accident.
As for those Motherships versus Carriers and others, Motherships are gonna cost huge resources and time to build. I'm guessing 1000 or more minerals and taking at least two or three times as long to build as a carrier. Blizz always tries to balance things (sometimes with mixed results, but still...) So as always, it depends on who you're fighting and what type of battle it is. On a huge map with plenty of resources, it's gonna be a long drawn-out battle, so I can see taking the time to build one or two way back behind the lines. On a smaller map, no way.
