Another benefit is that you don't develop that popeye-like, huge forearm like Rod Laver used to have. You maintain a more symmetrical physique.
Down side is that you don't have a free hand to do the following:
toss a ball to serve, or feed (tennis ball or sandwich)
take the racquet back, or change grips
put a ball in your pocket, or take a ball out of your pocket
adjust your balls
wave to your friend who may be on the next court or may be walking by
pick your shorts out of your butt (applies to Nadal, mainly)
signal "out" shot by raising one finger
give someone the finger
adjust your headband
do common finishing moves after great shots, ie air guitar, rifle shot, makin' popcorn, baseball home run swing, air banjo, Chi Chi Rodriguez sword fight, etc. (would assist with the following finishing moves: Darth maul saber battle, drum solo, lobster man, giant scissors, doubles crutches, snow shoes, biggest earings on the planet, and many, many more
if you are Italian, talk or communicate in any way
Also, on the follow through, you might clang the racquets together sometimes (known as "cymbaling" in the two racquet community).
Also, you might be called for a "hinderance" if you are at the net and the other person has a sitter, and you wave both racquets in the air, trying to guess which way the ball will be hit.
On the plus side though, on the above sitter example, you can cover your face with one racquet, and "other areas" with the other racquet. No need to bail out.
Plus if you drop one racquet, you are still in the point, although you might have to hit a backhand
Like anything, there are pluses and minuses. Who can think of others?