I think that the advantages of using both hands with equal ease is quite evident whether it be sports, at your work or while you are doing your household chores.
An obvious advantage is using both hands to write or draw with both at the same time.
I would be focusing on the disadvantages that may be faced.
1) Ambidextrous people are more prone to suffer from Synaesthesia
2) They are likely to possess (not necessarily) the LRRTM1 gene which
is linked with Schizophrenia (reference).
3) They score lesser in intelligence testing (reference).
4) Mixed handed children are more likely to suffer from ADHD
5) Mixed handedness is associated with greater age-related decline
6) They are easier to anger and influence emotionally (reference).
There are quite a few studies that have pointed out that it may not be all that good a condition but Leonardo da Vinci who was probably the most diversely talented man who ever lived is said to have been ambidextrous. He apparently could draw with one hand and write with the other at the same time, so I would have to say that the positives of the condition are extremely good (reference).
This probably qualifies as a second question :-D but I just saw a thought by biogirl in the comments section on "whether our ancestors used both the hands and using just one hand is recently evolved" and she has a point there. From a study publlished in 1977 I quote.
The predominance of the right hand over the left was also reported
among Egyptian art forms 3,500 to 4,500 years old, where the ratio of
left- to right-handers was 9:111 and 5:100, respectively. However,
going further back using the "Draw-a-Man Test," found that ancient
paleolithic man, from 1,750,000 to 8,000 years B.C., was probably
either more ambidextrous or that there was a greater proportion of
left-handers than there are now. (reference)
Also from a study titled "Evolutionary Back Grounds of Human Left Handedness", the authors concluded that
For most manual tasks, especially those tasks involved in competitive
activities, increasing performance by the specialization of one hand
is certainly adaptive. Aggressive interactions are responsible for
fundamental selection pressures acting during primate and human
evolution. The higher prevalence of right-handedness might well be due
to a previously existing cerebral bias. But the specialization of one
forelimb leading to right or left-handedness is better viewed as the
result of natural selection. (reference)