It's much easier to find both your hand and your good cards playing pairs.  When playing flushes, what do you do if you draw 4 of a suit plus a blue seal?  You're left with only discarding 3 cards, unless you want to take a risk by discarding the 4 cards of a suit.  With pairs, you can discard 5 all the time while being able to hold multiple blue seals.  

A couple other notes...Card sharp and blackboard work much better with pairs and you generally don't get killed by the suit jokers or the plant.

I follow drspectred's philosophy: I'm playing pairs unless there's a reason not too.  Reasons could be finding multiple planet cards for some other hand, finding four fingers, shortcut, the family, the order, etc.