MM mail is everything but an advanced mail account...Just one example among others: How do you mark several emails as unread??? You can do it one by one only...In Gmail you can select 10 mails and mark them all unread.
MM mail is just a joke...It is like a mail account for beginners...
I believe you are referring to the web interface. Can't you shift-select or CTRL-select your emails and then click on the Actions button on the top-right hand corner of the MM interface and select Mark... ?
Am I missing something?