Depends on the tablet. My current tablet (an Asus Transformer running Android) has batteries that last for about 16 hours of use, so I never switch it off. It doesn't have 3G, though, just wifi - but that is all I need. I can use my phone as a wifi hotspot on the few occasions that I need web access via 3G.
I don't use the tablet for all my Internet browsing - some things are easier to see on a PC with a big screen. But I do use the tablet for many Internet tasks, including email, watching online videos, various apps, etc. I also use it to watch movies, play games, and read ebooks. It has replaced my old work laptop as my mobile computer.
My first (cheapie) tablet only had one or two hours of battery life, and only had a 7 inch screen. So I didn't use it on the web much - just for reading ebooks.