But the NRP's partner IS financially responsible as they are contributin towards NRP's household costs: rent/mortgage, gas/electricity, food expenses, etc... which the NRP's children all benefit from. She doesn't just move in and not put towards living costs.
Additionally, she will be putting her earnings together with NRP's after his tax, NI and maintenance reductions, and the is their family pot of money- so clothes, additional expenses for NRP's children, clubs, anything else they will need. That money after all essentials are paid for is also their disposable income to do what they want with. Her earnings are not to be used as a top up of maintenance her partner has to pay. Why should she subsidise TWO households?
And the benefits thing. That's just how it works. If you choose to have a new partner it can have pros and cons. The partner of the nrp could have her own children and then her benefits get cut due to meeting a man with children. Similarly, an rp's benefits get cut when she meets a man who works.
I have never agreed with maintenance being reduced for stepchildren. I.e. the amount a nrp has to pay shouldn't go down when he meets a partner with children. And just as strongly, I believe maintenance shouldn't be increased if the nrp has a new partner as her earnings are completely irrelevant for maintenance purposes- just as RP's partner's earnings are completely irrelevant.