Thankfully the computer hardware industry is finally moving away from mechanical switches in keyboards and peripherals such as mice, however many devices, and especially the cheaper ones, still use them. Why they were ever used in the first place is beyond me, but i suspect it may have been due to the large selection of readily available micro-switches on the market and their low cost.
One problem with mechanical micro-switches is that they wear out eventually (and sometimes "eventually" can be a couple weeks). Another is that the contacts bounce when they slam together, thus a filter is required so that the computer sees one signal instead of several. The hard-core first-person-shooter (FPS) gamer, or those susceptible to marketing hype, pose another problem, that being the extremely short delay caused by the "debounce" filter having to process the signal in a genre of computer games where reaction time is crucial. This concern seems misplaced however if the anti-bounce filter sends the first switch contact to the computer as soon as it's detected rather than delaying it and, actually, this concern may be invalid regardless of how the debounce filter is implemented. I think the concerns of the hard-core FPS gamer regarding mice and keyboards would be better directed at the complex USB server-client protocol and its inherent polling delay verses the dead simple PS/2 connector which is making a comeback for this very reason.
With the potential exception of delay, optical switches that use light to detect a button/key press solve these problems. In theory a good optical switch would never wear out and no filter is required to eliminate contact bounce, though apparently a similar filter is still typically used to prevent certain switch events from being misinterpreted by the computer. Furthermore, that light must be detected and converted to a signal that the computer understands and i would suspect that the resulting delay may be even greater than that of a mechanical switch where the debounce filter sends a signal to the computer as soon as a switch contact is made, thus why i think the delay concerns of the hard-core gamer regarding mechanical switches may be unfounded.
So, if i were a hard-core gamer, i would be considering PS/2 mice with mechanical switches which use an "eager" hardware debouncer rather than USB mice with optical switches. For the rest of us however, the optical switch is long overdue!