You are so wrong. You know patents can be bought and sold alone right? Do you really think they bought a manufacturing sect and make a OS that can be implemented, but have no plans or desires too? They just wanted the patents? No sir, I disagree to the very core of your statement. I do think the deep patents were a driving force though.
And I'm a firm believer that while making their own devices might benefit Google, they were not looking for an OEM. In the disgusting patent world, they saw a struggling Motorola with a treasure chest of patents, and said "let's do it". In the short term, it benefits Android. Should later on Google decide they want to make their own line of phones directly, they CAN.
Google does this all the time, buy a company, and maybe down the road they figure out a use for them. Only in this case they gain both long term and short term benefits. My bet is that Google will be the absent parent company, Moto will operate as they are now (maybe even Blur intact), but should Google want something down the road, they can have it.
Also explains the canned responses from HTC, LG, and Samsung. Google explained, we aren't your competition guys, just doing this to protect you.