First off, alphas are almost by definition non-playable. Although I guess this doesn't make them lose all their interesting value, you should realize that you're not going to get much more out of it than watching random video's of it. Oh wait you are: the video's will have considerably less bugs.
Second, I've developed a passionate hate for this business' method of releasing things under false premises. Since I feel like ranting, here goes:
Demoes these days are absolutely crap. They are random excerpts from a game that offer no introduction and hence are hell to get your head around, usually just annoy the people that end up buying the full game afterwards since halfway they suddenly find they've already played that part and all the surprise is gone, and generally offer nothing whatsoever that actually makes you want to buy the game. Most demoes these days serve only one purpose, and that is to hold off the whining of fanboys who have already decided they will buy the game when it comes out, by throwing a half-assed bone at them. People who want to find out what the game is about to actually decide are greeted with what I've come to interpret as a giant "Fuck you.". Especially since the coming of the consoletard generation I generally find demoes downright insulting.
Recent attempts to keep people interested (or perhaps to stop the bitching from folks like me) have been to basically take what they previously called a demo (which is already an inappropriate term) and label it to up the interest - "open beta" is very popular in this respect, for example. It's a smart move from the company's point of view but beyond a business strategy it's tasteless. To further fuel my personal flames of discontent, gaming companies have reached a new low point by releasing 'demoes' which aren't even free anymore. In other words, to put it bluntly, you get a half-assed product, hardly worth any form of consideration at all since usually it's nowhere near a finished product (under the pretense of it being a 'beta') and instead of using it as a method to win you over to play the game, they actually make you pay for it - and then continue to ask the full price for the actual game!
The most horrific example in this respect is Gran Turismo 5: Prologue. Hardly anything more than a demonstration of what the full GT5 might someday perhaps be capable of, it was sold as an actual game to hold off the die-hard fanboys for another couple of years since Polyphony absolutely sucks at teaching their personnel how the concept of a fucking deadline works. It's effectively one of the most strategically brilliant but PR-wise almost criminal acts of misleading consumers in order to make everyone forget that in actuality the entire business is just too lazy to put a little work in an actual demo.
Today's output is so horrifyingly bad that I might even be tempted pay to see a proper demo. It's almost ironic.
So, based on that wall of text (modestly small compared to my track record) I can honestly say that I'm fully against the idea of any form of preview to a game that you have to pay for. Doesn't matter if it's a demo, beta or even an alpha. If the industry can't even put out something that can decently please anyone in the form of a free release, it has no right to ask people to pay for products that are worth even less attention. Shame on all the companies that have taken this route, how dare they.
Closing comments: although this is in no way a direct attack at Introversion, I do believe that indie companies should know better than any other kind what they need to give the audience to please them. Money troubles don't count as an excuse to me. Selling other random 'merchandise' like releasing source code is a great move, one I have no problem with and in fact even encourage - but releasing an alpha for payment will feel to me like I'm being spurn as a gamer.
Third: IV won't do this anyway so don't get your hopes up. Instead you might just want to be around for the beta sign-ups and you may be able to not only get your hands on a preview version without even having to pay for it - you can even contribute to its development.