Let's turn this around. Does saying "thankyou" publicly do any harm? I don't think it does, and so it shouldn't be discouraged.

Nope, of course not, its nice to acknowledge that we're all human being interacting electronically, and this shouldn't be discouraged, but let's not forget either that just like in real life, when conversating with a group, and you're trying to convey a general public thank you, you don't walk up to each individual to repeat your identical thank you message -- you might spend half the night trying to thank everybody, do you really have the time?

It does no harm, but depending on the number of participants in a thread, its tiresome to keep thanking folks personally -- and it might very slightly increase the noise level of the thread