I do hope it will always stay optional though, because otherwise it might very well conflict. But if it stays optional.. who will install it? Isn’t it more logical to use a more global mouse gesture application?

Sure, but what about people who hop between computers on a day-to-day basis? For example, think of students at a university who frequently check their mail on different computers throughout the day. This would give them the same mouse gestures from any computer they use.

Mouse gestures sounds good, but won’t it conflict with browsers’ gestures?

Probably, but that’s assuming your browser has mouse gestures. A majority of the population probably don’t have any mouse gesture capabilities in their browser.