I would say not to buy a house with someone when you haven't decided whether or not to get married yet. I would go further, and say not to do any sort of financial merging. Multiply this by a factor of ten if the other person is bad with finances.
As others have said, moving in together can cause huge problems if you don't share exactly the same views about what it means and how it will be done. What can happen is you end up with marriage-lite. You've got the trappings of domesticity, but you aren't actually married, and your partner hasn't actually made a commitment to anything more formal than cheaper rent. If you're happy with that, and don't need anything different, that's fine. But if what you really want is marriage, and you're settling for living together hoping that he'll change his mind, you can easily end up worse off. You can end up with all the hassle and legal troubles and heart-ache of a divorce, but without ever getting married, or having the protections of divorce law for things like division of property. (And to ward off comments - common-law marriages are not recognized everywhere, and even when they are, there are rules for what it means and how it applies. Simply living together, or even living together in a house you bought together, is not necessarily sufficient.)
It's also much harder to break up if you're living together. So it's easier to stay one more year, or two more years, or ten more years, hoping he'll propose, because it would be so hard to find a new place, or you can't afford a place on your own. So you end up in a relationship that is being decided by someone else, with no choice for you.
And it is really, really difficult to take a hard look at a relationship and say "This isn't what I thought it was or hoped for. I need to move on." Romantic relationships in particular have very strong emotional components, and it's incredibly easy to lie to yourself, telling yourself what you want to believe, rather than what is true.
I think most people with any romantic experience at all have done this, or have had to help friends who are doing this. It can be the hopeless crush on someone who isn't interested in you, where you interpret anything they do as a sign that they are falling for you. It can be telling yourself that really, he'll leave his wife for you. It can be staying in a relationship for way too long, telling yourself that he's going to propose soon because he's grinning funny, or he's hiding his credit card receipts (must be a ring!), or is taking you to an especially nice restaurant for dinner next week. Or telling yourself that he's really not a nasty person and doesn't mean to scream at you, he's just really stressed at work and you shouldn't have provoked him. Or that he gets along with his nieces and nephews really well, so he's obviously going to change his mind about having kids after you get married and he gets used to the idea.
But generally, if you're conducting a relationship based on the fantasies you tell yourself rather than the reality that's there, at some point reality catches up with you, and it hurts way more and is much more messy than dealing with the truth of the relationship head on in the first place.
