It just strikes me as very old fashioned thinking to not take account of the fact that most households have 2 earners
You must be young if you think it's old-fashioned. We only changed from doing like that in about 1989, I think.
A change back would make it less worthwhile for lower-earning spouses, usually women, to work. Even if they only have a minimum wage job, it would quite possibly be taxed at the higher rate.
When I'm Prime Minister, we will go even more the other way, benefits will also be done on an individual basis, not household. That will mean adults will be able to improve their circumstances by living together, sharing expenses, rather than getting more money by living apart, as is currently the case.
Just googled the following, this was in Nigel Lawson's budget speech in 1988.
The present system for the taxation of married couples goes back 180 years. It taxes the income of a married woman as if it belonged to her husband. Quite simply, that is no longer acceptable.
This is a matter on which there has already been extensive consultation. The time has come to take action.
I therefore propose a major reform of personal taxation, with two objectives: first, to give married women the same privacy and independence in their tax affairs as everyone else; and, second, to bring to an end the ways in which the tax system can penalise marriage.