Jesus you again. It’s not called a “cohabiting partner visa” first off. It’s a partner visa. You only have to cohabit while in the US.
No, as others have pointed out, this is incorrect. You need to have a provable history of co-habitation. Rental contracts, council tax bills, driving licenses at the same address - that's the kind of thing which you need to take along to your visa interview at the consulate. They may or may not ask to see it, but you need to have it to hand. Then, when you're granted the B-2 visa, the officer must annotate it as 'cohabiting partner' or you may experience further issues when you try to actually enter the US.
People use the B-2 visa for many reasons. It's not called a 'partner visa'. It's a non-immigrant visa for people travelling to the States temporarily for tourism, pleasure or visiting, which can be used by cohabiting partners of visa-holders to secure a longer-than-usual (but still temporary) stay in the US. As well as cohabiting partners, people also use it if they're want a holiday there but are ineligible for the 90-day VWP (eg they're from a country without the VWP, or they have a criminal record which means their ESTA gets denied), or if they're eligible for the VWP but want a longer stay.
The last one is generally only issued to retirees, or maybe people with exceptional plans like hiking the Appalachian trail or something. Otherwise, US immigration considers 90 days sufficient for the average working person's holiday. Asking for six months suggests you're going to work under the table or try to live in the States.
And the real pisser is that getting denied for a B-2 will probably prevent you from visiting the US at all for several years: having a visa denial on your record means your ESTA won't be approved. So this is definitely not a 'make an application, you've got nothing to lose' situation.