I usually only play as one of the sides in an RTS. What I would object to is being forced to play as a side you don't want... (e.g. in AoE3, you play as who you're told, I'm midway through the second act and I was desperate to be the British, but have had no luck.)
My worry is that they decided the three campaigns would be too large for one release, but are they large because they are filled with enjoyable and interesting missions? or will we have three times the missions, but lots of repetition?

People are complaining about one game + two expansions costing too much spread over 3 years or so, you can bet they'd have an issue with being forced to buy them all together at the same time. Plus, this way you can get to the multiplayer side (which is the meat of the game, anyway) without having to wait years for singleplayer stuff that you may not even care about.
SC1 and WC3 didn't have a 30 mission non-linear campaign with a metagame for any race, much less all of them, so if SC2 fails in comparison to these "other games", those failed even harder.

I've heard of some of the levels that they have - and they all seem to be interesting and unique.

Most campaigns are dull and many if not most who are into multiplayer play them only enough to get their bearings before firing up multiplayer, while many single players play them only enough to get their bearings before trying skrimishes or extra maps. Heck, a lot of people who are into multiplayer don't even read the manual and just jump into multiplayer as soon as they install the game, not even knowing what the commands are yet. Campaigns are an afterthought and/or quick training ground only to a very high percentage of players overall.
You can't blame people for not paying that much attention to most campaigns either. Many missions are unimaginative and artificially restrict the fun part of playing -- trying out your creativity any way you like. They're often repetitive and little better than filler. If six or ten of them are bad, I have no idea what the point is of having 30 ... or 90. Ugh.
My guess is Blizzard is doing typical Hollywood speak, saying they just can't release it as one game because of the elaborate missions they "just absolutely must have," even though they could obviously design as many or as few missions as they like, because they simply want to sell more boxes.
That's how business works. They may want you to impute grander or more sainted motives, but c'mon already. If you want to believe it, that's up to you. But to me the shortest distance between two points is a straight line, and there's a reason they don't call it the gaming art. It's the gaming business.

Yeah, I know, I am pretty much alone - well, me and one of my cousins - in that I mainly play for the campaigns and getting the story out of it, like an interactive book or movie.
And yes, painting a picture that people would want to buy, or composing a song or writing a book is business as well, that doesn't make either of them much less of an artform. The thing is only that our favourite pastime (games, that is) is a combination of several of the traditional art forms - acting, writing, visual, and audial arts.
Though I agree, everyone has their own beliefs, and I believe that the reality is a mixture of many decisions in many directions - "what makes us most money", "what is most fun to play", "what looks best", "what holds best up to the players' expectations (or surpasses them)". It's a tight line to walk and ultimately there are sound reasons why gamers all over the world holds the company in such high regards - and sound reasons why the gamers of the world are split over which companies they prefer to support. No company is a collection of saints, but neither is no company a collection of devils, we're all human, whether we're art directors, coders, or CEO's.
I'm pretty sure I didn't meet any of your points as I zoned out for a minute, but bear with me.

http://www.youtube.com/watch?v=g_6IVesXjKg
thats the video that I was talking about

Just thought I'd update to let you all know that while you can play offline, Blizzard has recently stated, in a follow-up with Kotaku, that features will be disabled if you choose to play offline. They didn't specify if that was multiplayer, so I'll have to assume they're being careful with their words because it will disable single-player features.

Makes sense right? Lets be online to play single player!

Maybe, or it's "your settings and replays are stored online with your profile so that you can access them when playing from a netcafe or at a friend's computer, thus you cannot change hotkeys or record games when playing as guest".

Well, I don't particularly care about the motivations of other gamers, but I'm a single-player gamer only.
I play RTS games for the single player campaign (playing AoE 3 atm for example), and do enjoy a well made campaign.
If they want to make a multiplayer only experience, noone is stopping them. Many folks would be very happy if they did (it would be faster to make, so they could have released it by now perhaps).
If they are going to include single player, then I would appreciate it if they thought about folks like me who are SP gamers.
For some reason this has really annoyed me, I know you can press "play as guest", but why can't I just have a shortcut to the game that avoid battlenet completely? I'm never going to use it after all...

This is a major inconvenience to all those who played the first and might not have the internet or something. Then again it isn't likely to happen and all it does is restrict a few features, but still.

I have seen the future, man... it's bright blue. Also, it tastes great.
Around here there are at least a couple places (for a lack of a better word: netcafe's) where you can pay to gain access to a computer and play various games (or just surf the web, LAN games are the main attraction though). Maybe LAN-cafe or gaming cafe would be better terms, I dunno.

Maybe you can, "sc2.exe -guestmode" or something may work.