An extra eye on one side of our head would be asymmetrical and probably beneficial as well.
How so?
Ten fingers (2*5)
Pentadactyl limbs are believed to be an evolutionary 'accident'. As to why we have two hands, see below:
two arms (2*1)
We evolved from four legged animals, four legs is the minimum number for easy walking (since you can maintain a stable tripod at all times - which is what the earliest known land animals seem to have done). Symmetry is the best for easy movement.
two eyes, ears
Two is the minimum number required for stereoscopic sensing.
and a nose precisely in the middle.
If you've got a body plan based on two of things, where is the logical place for something you only need one of? That's right, in the middle. Otherwise you'd de-balance the body plan, with the resultant constructive and mechanical difficulties.
And of course there are the decorations, such as completely symmetrical designs on butterfly wings. How did evolution come up with that?
Symmetry is almost universally attractive among animal species (including humans). Butterfly decrorative patterns are generally either display, or eye-mimic patterns. Display patterns will want to be symmetrical for the above mentioned reason, eye-mimic patterns need to match the eyes of those they mimic.
There is a good reason for the attractiveness of symetrry, perfect symmetry (as opposed to near-perfect symmetry) is very difficult to build, it otherwords it's energetically expensive, while being functionally minimally advantagous. Thus successful mate material will be able to spare the energy required for symmetry while less fit organisms will not.