Here's my opinion... is Mantra crucial? No. Is mantra helpful? Absolutely. Now here is my question to you... Why wouldn't you want to give you and your group every advantage you can? The mentality of "you should be able to get by without it so don't bother" boggles my mind. All I see when I read that is laziness and apathy.
When you're in a static doing endgame raiding, everyone should bring everything they can to the table to help the group succeed. If you're not, then why are you even there? If you don't care enough to maximize what you can bring to the group, why should you expect anyone else in the group to?
