The giants are the one thing I dislike about The Hobbit (luckily it's a small part!). From my very first reading, I wondered why they were there. They seem silly and out-of-place, and the whole business with them tossing boulders around during the storm is just like when parents tell their two-year-olds that the thunder is really "the angels are bowling". I rather enjoy Tolkien's fatherly narrative tone the rest of the time, but here it's a bit condescending. I try to rationalize the giants' presence in two ways, both of which have faults.
1) Perhaps Bilbo is just telling himself the silly "giants playing at ninepins" story to keep calm during the storm, which is really big and loud and frightening. He doesn't really believe there are giants out there, and neither do the dwarves -- it's just a figure of speech when they talk about them. Of course, when Gandalf mentions trying to get a giant to block the entrance of the goblins' tunnel, that discounts my theory because he's clearly being quite serious and saying it sincerely.
2) Perhaps "giant" is The Hobbit version of "troll" in the same way as "goblin" is The Hobbit version of "orc" (meaning that the one word is used mainly in The Hobbit and the other in LotR). After all, "ent" is an old word for "giant", and Sauron (or was it Morgoth?) made trolls in mockery of the ents. But Gandalf's statement messes up this theory too, because he wants to find a "more or less decent giant", but giants/trolls made by the Dark Lord can't be "decent".