If you ask me, it's about ease of use.
It's a lot simpler to portion out fractions of a package of DME - every try to get a half-pound of LME out of a Munton's tin? You can just snip the corner of a DME bag and tap out enough powder into a bowl.
Liquid yields also less fermentable product per pound than dry, if your storage space is limited.
Finally, liquid extracts don't have anywhere near the shelf life of dry.
According to Palmer:
The freshness of the extract is important, particularly for the syrup. Beer brewed with extract syrup more than a year old will often have a blunt, stale, even soapy flavor to it. This is caused by the oxidation of the fatty acid compounds in the malt. Dry malt extract has a better shelf life than the liquid because the extra de-hydration slows the pertinent chemical reactions. *
I prefer dry simply because of ease of use, but that paragraph should give syrup users pause. Many LHBSs don't turn over enough inventory to avoid long shelf storage; make sure you check the package carefully.
* HowToBrew.com Chapter Three.