For one I think Starcraft I was near perfect in terms of single player campaigns: Each one lasted only 10 or so missions, so you got new units with nearly every mission and before it might get boring you switched to another race and storyline.
30 missions in a row in one campaign just sounds like dragging it out. I know they're going to switch to a different race a few times in between, but still.
Also, I kind of doubt that the games will go for less than your usual game does: A lot of people will only buy 1 game for multiplayer, so Blizzard has to make sure they make enough money even if 60?% only buy 1 game instead of 3. So likely, you will have each game going for slightly less than what you nowadays pay for a game. And then you will have 2 further games going for the same. I can't even blame them, their last game must have had the best revenue/cost ration in history ever, so of course they'll be trying to forcefeed us a similar recipe. Depends on if we swallow it.
Personally I'd much rather spend normal full price for 30 mission game once and get a 3/4 full-price addon a year later that adds a second batch of 30 missions. I don't even need new units.