It's a combination of both for me, and I think the main reason is because I love the total experience of being a fan. I believe that the fan experience is two-fold:
1) To be able to engage knowledgeably in discussion, you need a thorough understanding of the canon. Drama fandom is interesting in that there really isn't a canon, but rather a canonical archive. The components of this archive are: completed dramas; actors and their roles; directors and their works. Without knowledge of all three, it's difficult to engage in some of the conversations that goes on here on MDL, for example, and I find myself marathoning shows so that I can catch up to some and participate fully in this fandom.
2) The second aspect is what I consider the beating heart and living soul of fandom, and that is simply living in the moment of a particular fandom. Midnight releases of books; midnight premieres of movies; standing in line for days for the latest Apple gadget or video game---this is the lifeblood of fandom, and this is what makes fandom so much fun for me. I love watching a drama that's airing and knowing that another 23 year old in Guam is also anxiously waiting for the subtitles to be uploaded. I love coming onto MDL, reading everyone's thoughts and reviews of the latest episodes, and tossing in my own thoughts. I love feeling like a part of something big that connects me to people I would have never met, albeit virtually. Granted, my enthusiasm for a show can diminish a bit if it isn't very popular, but more often than not I think to myself that I've found a gem of a show that no one appreciates but me.
However, I lean more towards airing dramas for a more practical reason: it provides a natural stopping point, i.e. the limit of the episodes released and subbed, thus curbing my addiction for the moment.