I dont use it at all. Not via the UserCP, not via Email, not by the guys wearing the brown shirts that come to my door daily. I know just by looking at a topic title, and the fact the forum has icons to show it as well, if i have posted in a topic or not. I dont have to be reminded if i have posted in a topic and to check the thread again. Since i go into every forum on the site just looking at the forum index tells me if there is new posts to look at. So subscriptions dont mean anything to me as they tell me something that i already know. I like redundancy, but that is over kill.
Just hover over the envelope in any forum index. It will tell you right there if you have a post in that topic. If not it will give you the Alt Text of the first post. Just going forum by forum you can easily tell if it is something you have responded to before or not. So why use subscriptions when you already going into the forum, just have to know how to use the icons to your advantage. So as you can see subscriptions are useless for me. As they only give me information that i already know. Since i dont have constant access to the site, and the fact we dont have a mobile version, i dont need email notifications. As if i am reading me emails, i am at my desk anyways and will be on site shortly to check the topics myself. See where this is going.
Topic subscriptions are for those that more or less dont come by daily and do check every forum. That isnt me. I am here daily and i know pretty much what topics i have read and which ones i havent.