I use my tablet for three purposes:
- Watching movies
- Reading books for leisure
- Reading books / PDFs for university (or otherwise studying)
Tablets appeal to three distinct primary markets: corporate users, content consumers and PC-replacement-users.
You're addressing an issue with content consumers -- I'm not sure what the issue is, as you haven't attempted to state your argument beyond tablets wasting time that could better be spent reading. While it would be wonderful if everybody enjoyed reading, that's simply not going to happen; furthermore, there's nothing wrong with choosing to consume other digital content such as games, movies, television shows and music. Many of these can be just as educational as reading a book.
Many people own tablets to replace PCs. These people probably never needed to fully utilise a PC in the first place, and what a tablet can do is all they've needed in the first place. I wonder if you would tell people that their PCs are a waste of time because you can also game and watch movies and consume other miscellaneous digital content on a PC -- I assume that you've posted this thread from a PC (or smart phone, or other device capable of providing you with digital content for consumption).
I have a smart phone, a tablet and a computer. I use all three for consumption of digital content -- for games, movies, music, podcasts and books. All three are entertainment devices, but all three also enable me to keep up-to-date with the world; and besides, the greatest form of entertainment (for me) is reading. Before the digital world, paper books were "entertainment devices".
I'm not often glued to my tablet, but when I am truly glued to it, it's because I'm reading for my own leisure or I'm reading a document for university. I'm not often glued to my smartphone, but when I am glued to it, it's because I'm on the phone to a classmate or a lecturer, or I'm listening to a science-y podcast. I only ever watch movies when I'm with company; I'd consider it rude to whip out my tablet and start reading a book in a guest's presence (but I'm an extrovert). To answer your question, I spend approximately 95% of my time on my tablet reading -- just over half of that time for leisure, and just under half of that time for education.
I agree with taustin, but I bit anyway, because I can't sing the praises of tablets highly enough - they're better than carrying two text books and a fantasy door-stopper! (And, if your eyes can take the LCD or you're not bothered by a transition between e-ink and LCD, they can even be better than lugging around more than one reading device in the same circumstance.)