It should be obvious enough that fine motor skills vary to the extreme when it comes to left or right-handed affinity between different people.
I'm almost completely ambidextrous when it comes to sports or anything involving or large movements, though I usually prefer my right for those things. However, I cannot use my right hand for
anything requiring fine precision like writing, art or using a mouse. I'm simply not physically capable of it. The left side of my brain developed the 'brute force' motor skills while the right side of my brain developed the fine motor skills.
More to the point, all high-end mice are only ergonomically designed for right-handed people, with the notable exception of the Razers, of which I'm not particularly fond, but may be forced to use. For someone that is using a computer for 90% of his waking hours (network admin), right-handed mice are extremely uncomfortable and unusable for long periods of time. Granted, an excuse to take more breaks from work wouldn't be a bad thing...