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Now, it was many years since I read The Hobbit (too many) but destroying the bridge makes a lot of sense to me. In fact, the only reason they built the town on the lake and not next to it was because of the dragon. Although the inhabitants of lake town didn't have any experience of dragons themselves, their not-so distant ancestors did and it was dire, as Smaug brought complete ruin to Dale.

Of course, had Smaug been able to land in the middle of Lake Town, destroying the bridge would've made no sense, as you pointed out. We'll have to assume he couldn't. I guess there would've been too many rooftops, towers, canals and such, for him to make a safe landing. Furthermore, Smaug would've been afraid to get wet and cool down, which would've made him less potent. If he tried to land on a rooftop it might have collapsed, causing him to plunge into the water. In fact, since Lake Town was built as it were in order to offer protection against Smaug, the constructors would have made sure there wasn't any place for him to land safely.

And if we assume Smaug couldn't land on Lake Town, it's clear why destroying the bridge makes sense. Had he been able to land on the shore and cross the bridge on foot, he would have smashed the town to smithereens and killed everyone foolish enough to defend it without any doubt. A known fact about dragons is their soft belly, and Bard was able to slay Smaug only because the arrow came from underneath. With his belly firmly on the ground, a great drake like Smaug is nearly invincible, and the defenders of Lake Town would stand no chance had he been able to cross the bridge.