Tablets are fine for typing-heavy work, just use a bluetooth keyboard with the tablet on a stand. This mode of operation isn't common yet, but that's because tablets are still relatively new.

Edit: to clarify, right now we're at a point where it's rare for a tablet (or smartphone) to be someone's primary or only computing device, but as they become more popular that will increasingly be the case, and then people will develop new habits and new modes of use to fit these devices into their work or their lives. Meaning that if people are using them at work then they'll tend to have some way to use a physical keyboard with them for the times when an onscreen keyboard won't cut it.