For me a forum address is first and foremost a cultural thing. If you are a part of the Bitcoin community, you should have a Bitcoin address associated with you.
And, it was also supposed to encourage the culture of embracing Bitcoin's power of low-friction transactions to express gratitude in the form of microdonations. It's not a huge success.
but when i check their addresses in blockexplorer, they invariably appear empty.
My forum address is not empty.
Let's just admit that technique doesn't work
It doesn't matter if people tip you or not, they should have the option.
and start actually doing something useful to earn some BTC...
The forum address is meant to be used to earn BTC for useful things that cannot be otherwise monetized. But I haven't heard of a forum address standing in the way of anyone doing useful things that can be monetized.