The main problem with using a proportional font for terminals is that so many programs use spaces and tabs to align text in different rows to better convey the structure of the information they present.
A secondary problem is that the control sequences that tell the terminal where to move the cursor are all defined in a grid-based coördinate system - if the terminal receives a "move the cursor down one line" sequence, its new position will be different depending on whether you choose to conserve the horizontal coordinate measured in characters or in pixels.
Of course, it would be perfectly possible to design a terminal whose control sequences made no such assumptions, but the DEC VT100 and its multitudinous offspring have been with us for a very long time now, and probably always will be.
While I program in a proportional font - given that code is what I spend most of my time staring at, it being readable makes a big difference - I still have my terminal sessions in fixed space fonts. Far too many things assume that it is being displayed this way.
Aquamacs Emacs (http://aquamacs.org) also lets you use a proportional font, and hey, it's emacs. It is, indeed, sometimes very nice, particularly when writing prose (Markdown or HTML or whatever), and particularly in version 2.0's new fullscreen mode.
A secondary problem is that the control sequences that tell the terminal where to move the cursor are all defined in a grid-based coördinate system - if the terminal receives a "move the cursor down one line" sequence, its new position will be different depending on whether you choose to conserve the horizontal coordinate measured in characters or in pixels.
Of course, it would be perfectly possible to design a terminal whose control sequences made no such assumptions, but the DEC VT100 and its multitudinous offspring have been with us for a very long time now, and probably always will be.