Well, it depends. There are times when I feel it is necessary to read the entire thread to compose a proper response to the topic at hand. I'd rather avoid appearing clueless as to what's going on. If for whatever reason I am unable to read the entire thread, I'll at the very least quickly browse through to get a main idea of where the discussion is. Such threads include RPs and debates in the MD section, where it is often required that most [if not the entire] thread be read.
However, some threads are not worth wasting my time to read. Said threads are either 1.) Quick response threads, like "What's Your Favorite Color?" or "The Picture Thread", in which no discussion or debates are necessary; all that is necessary is to reply to the OP, hence the application of the term "quick response." 2.) A simple forum game [excluding most RPs] where the topic is practically nonexistent and, if anything, all that is required is that the post above be read.
There are times, however - when I'm bored, have a lot of time on my hands, and nothing better to do - where I'll just relax and read the entire thread just for fun. Many interesting things can be found, especially in threads that existed before I joined the forums. One can also learn a lot about the ZD DGN community and get to know members better by reading certain threads in which they posted. I admit that longer threads can be a pain, though.
So yeah, it depends on the thread.