mrow - child vertical align

Hi,

In a mrow tag I think we need to have a vertical align attribute: top,
bottom, center or baseline - like rowalign.

If we have children of type mtext with different mathsize I think is better
to be rendered using baseline (also center could be a good choice ).
If we have a list of fractions in mrow is better to be centered.
...

What do you think ?
Do I miss something ?

regards
Ionel Alexandru
www.fmath.info