I can't see any justification for giving this to any low paid person just because they are married. What about all the single people on low pay?
But single people on low pay are already entitled to more in the way of benefits etc (as its only their income that is taken into account).
I was a single person, self employed, on low wage. I had housing benefit, council tax benefit etc. When I married my partner I lost all that (due to his wage), and he lost his single persons council tax reduction etc. Being able to get £200 odd back a year for that is hardly some huge advantage for getting married.
(Though in my specific case, my husband is actually a higher rate tax payer, so we don't actually get anything).
I do know people who would move in together but feel they can't afford it (because two single people get more from the state, especially if you each have children from previous relationships etc), and I think that's actually pretty rubbish.
But I don't think is tax break actually does anything really to address that problem - I think the government were very careful with this to make sure most people wont qualify (i.e. no unmarried couples, no couples where one is above average and the other below, no couples where both are earning average amounts etc). So they can oh-so generously give a tax break which sounds like they are addressing something on paper and doesn't actually cost them much.