Destroying the bridge is the smartest thing to do in terms of survival - you set up a defensive position, with no access from/to the shore & fill it with archers. This allows the rest of the population to escape in dribs & drabs by boats, landing at many different points around the lake, while Smaug is distracted in attacking the town. Having everybody trying to escape via the bridge would be idiotic, as they'd all be funneled onto the same part of the shore line. Bard & the defenders are setting themselves up as a diversion to allow their fellow citizens to escape.
Now, of course, that may not have worked out:
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Soon all the town would be deserted and burned down to the surface of the lake.
That was the dragon's hope. They could all get into boats for all he cared. There he could have fine sport hunting them, or they could stop till they starved. Let them try to get to land and he would be ready. Soon he would set all the shoreland woods ablaze and wither every field and pasture. Just now he was enjoying the sport of town-baiting more than he had enjoyed anything for years.
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but the point is they are fighting for survival against almost impossible odds. Most likely they will all die but they're making the best of a very bad situation. Quite simply, destroying the bridge is just marginally smarter than not destroying it.