I agree completely with both of the opinions offered. Intent is a tricky thing to discover. Of course, after the War of the Ring is over it is easy to see how critical Bilbo's involvement was; Gollum would have been found with the ring, sooner or later, especially as its power grew, and so by falling into Bilbo's hands the ring was given its greatest obstacle.

Based on Gandalf's behavior and his own words, I am inclined to believe that he never fully knows what is ahead of him. I have no doubt that he instinctively understands what needs to be done, but he is no fortune-teller. But this is not a deficiency; it is one of his greatest strengths. If he were omniscient, the pressure would overwhelm him. To accomplish his goal, Gandalf doesn't need all-powerful foresight.

This also gives him the opportunity to be a lot more fun and a lot less serious than some of the more all-seeing characters. It allows him to live in the moment a little more, which would explain his affinity for smoking, fireworks, friendly hearthside chats, spontaneity, and lucrative adventures. I believe that while he knew that Bilbo was vital to the venture, he didn't completely know why. He just knew that Bilbo could not be left out.

As the purpose became more clear, Gandalf was able to click into "serious mode" and get business done. But in those few precious moments here and there where he can have a bit of fun, relax, and enjoy a quality adventure with good friends, why not go for it?