Having gotten married more or less exactly in the middle of my PhD, I would say emphatically no, it is not a bad idea at all! (I expect you just meant 'during' by 'in the middle of' though. =] )
Regarding the 'meagre pay': at the time my PhD scholarship was about twice as much money as I had ever had in my life before, so I felt comparatively wealthy. We were not interested in having a lavish wedding anyway, but we did manage to afford a very nice wedding with 80 guests, paid for out of money saved from our PhD scholarships.
Of course PhD students might not be similarly well-paid in many countries (we were in the UK).
As for the 'burden of responsibility', I'm not quite sure what you mean by this, so I'll ask a question in a comment and wait until you reply.
Points to consider:
You will potentially need to lower your expectations about what kind of wedding you can afford, but if the point is to achieve the state of being married, I don't think that's a big problem. You can always throw a big anniversary party at some point later when you have more money.
Potential name change (as mentioned by F'x): If a name change is going to be involved, it can be very convenient to have this happen before one has any publications, which is a lot more likely to be the case during the PhD than afterwards. (I did change my surname, and had no publications yet at that point, so there is no problem with using my new surname for academic purposes.)
Honeymoon: There's a lot more chance of getting enough time off for a proper honeymoon during a PhD than once you have a job, although this is going to depend completely on your supervisor, of course. [Added because of F'x thinking the honeymoon could be a problem: My supervisor allowed me a month after my wedding.]
Companionship, as you mentioned: This is obviously especially relevant if you are from a culture/religion where living together before marriage is not usual. Definitely having 'an intimate companion' during the PhD journey (not that mine was that long or bleak - but my husband's was more so) can be a huge help, and I don't see any reason for a couple to wait until after a PhD to get married if they are ready now.
I may add more later if I think of anything else. There is more I would say if I wasn't trying not to stray too far into 'discussion' or 'off-topic'ness.