If they don't have Brock/Rock for the title then they would have to negate Rock's promise of aiming for that goal which some casual would probably forget anyway unless reminded. If it's non-title WM29 will just be another show with main events that are bigger than the title match and will receive more build and attention. The WWE Title needs more relevance on the grand stage and having Rock/Brock fight for it could give it that attention. Although Brock/Rock don't need the extra attraction/element but maybe the WWE title needs it.
Here's the other problem with that... and I mentioned it already, but what happens after Mania?
The WWE Title gets that big special attraction, but where do they go with it after WM? Just vacate it? First of all it makes whoever wins it not feel anywhere near the level of Rock/Lesnar. It's just something where whoever wins it won't feel like the true champion and the WWE Title will just fall back down to the level it's at now. If Rock/Punk for example happened, and Punk beat Rock, it would be a huge match, would put the WWE Title in the spotlight and make it feel important again AND (and this is the part missing from Lesnar/Rock) it will continue to be very important as a current guy is holding the title.
If Rock stayed with the WWE for a few months and then lost the title to someone, then I'd be fine with Rock/Lesnar being for the title. If he just wins it just to vacate it... well... it's just a temporary solution to a permanent problem. And since Rock more than likely won't be sticking around past Mania, that creates a lose-lose situation if Brock/Rock happens whether for the title or not. If for the title, the problems are listed above. If not for the title, as you mentioned, the WWE Title once again feels lost in the shuffle and not as important.
This is why I say Rock and Lesnar shouldn't face each other. One of them can be in the high profile streak match, while the other competes in the title feud and puts over whoever they face... putting over the superstar and title. And as I said, since Rock said he was basically going after the WWE Title on his return, it makes sense for him to do that, while Lesnar faces Taker for the streak.
And in order for the streak match to work, Lesnar needs to destroy HHH and actually WIN the match, as well as any other matches he has. His loss to Cena hurt his credibility, but it's not damaged beyond repair and if they do with Lesnar what they should've done in the first place, that loss will seem like a far away nightmare.