think the main advantage of 29ers seems to be going over obstacles like roots etc, makes climbing over them easier for example
you get a smidge more traction and grip too
disadvantages are, like you say heavier wheels that flex a bit more than a 26". Plus components (tyres and forks) are still a bit limited.
27.5 seems to be getting popular too! I'm waiting a few years to see what happens, but I reckon 29 will be for xc, 27.5 for all mountain and 26 for downhill