[BigBalli]

The vast majority of websites that disable it, they're actually doing it to provide additional functionality which would be lost otherwise. Seems like using a sledgehammer to snap a button.

[eek2121]

I have no clue what websites you are visiting, but the ones I have used typically either do it to disable/customize copy/paste or use it to track your actions. They often make too many assumptions about the browser in general.

[Lerc]

Left button draw, Right button erase

Left button rotate, Right button pan

Left button gun, Right button grenade.