While I agree that Metallica at Glastonbury is a strange choice--and probably one that will bring in a crowd that will clash with the usual Glasto crowd and cause headaches--I don't agree with the assertion that Lars Ulrich is a terrible drummer.
All of the members of Metallica are accomplished musicians. You may not like individual styles or choices, but you can't escape the fact that these guys can play. I just don't see how their set works in that setting. It works great everywhere else--in the context of their arena and stadium shows, people get what they expect and more. They play their guts out.
I think Mogwai are looking for a little of what Trent Reznor and Pat Carney need--attention and love.