Well I use both Instapaper for reading articles later and Pinboard has a read later bookmarks section (which I use mainly for videos) - both of which work best when you have decided you have time to spare.
Where Reminder Bear and Changedetection.com come in is to remind you to check out something (have you considered buying these fancy headphones on amazon but wanted to wait a few days to see if you still wanted it then) or as soon as things have changed in case of changedetection.
I have used Reminder Bear to check back for answers on certain Stack Exchabge sites I don't visit otherwise and also on forums that I wanted to follow a discussion but not become a member. So if you don't mind using another tool there's definately a niche for people.
Hope this clarifies things