For example: why do we celebrate Christmas? Tradition strikes me as the major reason here. Moreover, it seems a perfectly adequate one. After all, some traditions are important to us. As Russell Arben Fox explains:
Holiday traditions are not just a lot of fun; they're a way of marking time, of moving oneself and one's children through life with other people and sense of history and all the rest. Not that any of us necessarily draw a lot of moral sustenance from Halloween, but the harvest imagery, the stories, the jack-o-lanterns, the costumes and parties and tricks and candy--they all add up, and add something to living through another October together.
New Zealand has only recently imported Halloween, but - thinking of Christmas instead - I can vaguely identify with his sentiment.
Now, I don't think tradition has any intrinsic worth, but if it had a general tendency to maximise the good (e.g. through its importance to us), then it would be reasonable to appeal to tradition as a 'rule of thumb'. So the real question is: does upholding tradition tend to maximise the good?
There are some arguments for classical conservatism which suggest this may be the case. To quote Edmund Burke:
We are afraid to put men to live and trade each on his own private stock of reason; because we suspect that this stock in each man is small, and that the individuals would do better to avail themselves of the general bank and capital of nations and ages.
(Collective intelligence, in other words, or an expanded version of 'the wisdom of crowds'.)
One could also argue from the idea of society as an evolving 'organism'. The existing social framework has withstood the test of time. It certainly isn't perfect, but at least we know it works. Revolutionary changes, by contrast, upset the social order in potentially disastrous ways (e.g. China's Cultural Revolution). Of course, this doesn't imply that we should rigidly rebuff all attempts at reform; stagnation isn't exactly adaptive either. But I do think it provides us with good reason to prefer gradual reform, and be wary of overly ambitious or 'revolutionary' attempts at social engineering.
So I think I should temper my original view somewhat, and concede that tradition can provide us with reasons after all. But they're certainly not absolute (or even moderately strong) ones. If one has good evidence that a particular tradition is pernicious, then of course one should cease to support it. The tricky question is, as always, where to draw the line. Despite the above arguments, I'm not convinced that tradition is a very reliable rule of thumb, so I'm inclined to grant appeals to tradition only as very weak reasons, easily overridden by other considerations. But I'd be interested to hear other people's views here.