I am going to go out on a limb here and say that 1 handed seals are probably not a good thing.
Going to spoiler this just in case anyone doesn't want part of the story for fairy tail told
My point here is that jutsu created with 1 hand may not be as powerful as that created with 2 hands, so the reason we don't see it often is simply because, ninjas only use them in situations where they can not use both hands for 1 seal.
examples
deidara was probably making clay bomb in 1 hand when he was going to detonate the bombs with 1 handed seal.
Zabuza had kakashi in a water prison which was much more important to him than killing the genin brats. so he used 1 handed seal to make the clone
Even in the case of haku, she/he needed to use 1 handed seals to combine the 2 elements, its not like she had 4 hands to make 2 sets of seals.