First of all, Rock is not full time. He has the same schedule as Undertaker and Brock Lesnar. Neither of those two wrestle when they come back too, because all the focus is on the main match.
Secondly, maybe The Rock doesn't want to wrestle all the time. It could be in his contract that he only wrestles specific matches. I don't think when Rock said he would wrestle John Cena or CM Punk, he also said he was going to wrestle Dolph Ziggler and Cody Rhodes on the side as well.
Thirdly, it's all the more meaningful and gives more importance to the main match. Throwing away free matches on Raw with the likes of Dean Ambrose etc, is just going to take away from the main match.
I think WWE shot itself in the foot by centering its product around The Rock recently. Bringing him back is going to be bad long term for WWE. They should have brought him back in a lesser capacity. Secondary Story lines, or use him to push over mid card wrestlers who need a break. Putting him on the title is stupid, because he will be gone after WM.
Lol, I beg to differ. Centering the product around one of the three biggest stars in wrestling history is a good idea. You don't get any better then The Rock tbh, the next would be if Stone Cold were to come back, but that's for another topic. Also why bring him back just to put over some mid-carders? He's not talent enhancement, he's a main event star and should wrestle with the top main eventers in the company. That's where the money matches are. I don't think people are really going to be enthusiastic about paying to see The Rock V some nobody like Damian Sandow or something, because that's a shit match and beneath The Rock. Rock Vs Punk or Rock Vs Cena etc are the matches people want to see.
As for putting him in the title scene. If people are only thinking about Rock only holding the title till Wrestlmania, wait until after Wrestlmania, because it will most definitely be John Cena again and I don't think really care about that. At least with Rock involved, people will care. Afterward, the title will once again go back to being on the top stars once more. Nothing will change either way, so why not do it, it won't hurt the product at all.