A tablet can replace an old PC with a deficient interface, at least by the way I use a PC. I use many keyboard shortcuts in any program which supports them, prefer a full-size keyboard with a numeric keypad and proper F1-F12 keys, prefer a mouse with at least 9 programmable buttons, have two rows on my Windows taskbar at work to support the multitude of windows I'm working on, and make full use of two >21" screens, to the point that I would love to have 3 or more monitors available. (Alas, many programs have poor multi-monitor support. For example, some dialog boxes in Pro/Engineer will only
open on monitor #1. Others will open on the correct one, only to leap over to the other monitor upon receiving any manner of mouse click.)
In any case, I find that using a tablet is like attempting rapid surgery while wearing mittens filled with cold gelatin. The primary benefit for me is the portability. I do have a Windows tablet that's running a dual-core i5 processor with 4GB of RAM, but it's still fairly sluggish for my tastes.
Different tools for different jobs, I suppose.
And I will also offer that I'm something of an outlier so far as computer usage is concerned, at least when compared to people at work, or family, or friends. It's like it's my natural habitat.