The facts:
https://www.gov.uk/marriage-allowance
Some people might argue that it's unfair that a person can't transfer ALL of their personal allowance to their partner (if they've got no income of their own).
For example, if two people living together each earn £15,000 they'll get taxed on £8000 of their combined income (because they'll each get £11,000 tax free). However if only one of them is earning, receiving £30,000 per year, they'll get taxed on £17,900 of their income (because the earner gets £11,000 personal allowance plus a further £1,100 allowance transferred from his partner under the rule which you're objecting to).
So, with the same combined income of £30,000 p.a., the second couple will pay £1,380 more tax every year than the first one does.