Yes I could do a 9-5 but wouldn't be prepared to while dh carried on in his career because it would mean doing virtually everything at home as well as having a job. I've no desire to do that just so I can generate more tax. So the reality is I could work but I would expect dh to cut back on his full on job too. So if we both earned between us what he currently earns now we would pay less tax and claim cb thereby costing the country more. Which is what I said at the start of the thread when the assumption was made that no sahp makes a contribution to anything other than their own family.
I think it's perfectly valid to say that you choose not to work because you don't desire to do that as well as housework etc. If it makes your own life and that of your DH easier, then fair enough - you are obviously adding value to your own family.
However, I still don't see how you are enabling him to do his current job or contributing to his income. You are reducing the pressure on him, yes, but that's different. Housework really doesn't take that long if you do it efficiently, especially not if split between two people. Your DH could still earn what he earns now, but you'd both have to work a bit harder.
Like I say, I think it's entirely reasonable if you choose not to make your lives harder than they need to be. It's a perfectly rational choice, and I'm not of the view that women should go to work. All families should organise themselves in the way that works best for them. I just don't understand why people feel the need to justify their choices by asserting that they enable their partners to earn more when this is very rarely the case.
For many families, there will be significant benefits to having one parent SAH. We chose for DH to SAH for a short period ourselves. However, the benefits cannot necessarily be measured in economic terms, and generally, the benefit is to the individual family rather than to society as a whole. Consequently, the state is unlikely to encourage this choice through tax breaks.