I Agree, Timing Belt is bad idea, and I guess could be considered a 'hack' But it works, not well, but it works. Other then small machines like 3d printers or laser cutters, I would never use a timing belt for anything like a router or spindle because the lateral forces would just bee too much for a belt.
He did mention he wanted an accurate machine, so I threw the options out there, but he also mentioned using aluminum extrusion. Yes, getting ground steel channel/square/rectangle would be much more flat/accurate than extrusion, but that can be very expensive to machine, even for short lengths.
Ultimately, when I have my machine built, I will have the base made from steel, taken down to a local machine shop who has a large enough machine, and have the whole surface ground flat, then apply the aluminum extrusion. Any other discrepancies in flatness from the aluminum extrusion (if any) will be taken out by surfacing the spoilboard. Yes, my machine will just be for wood/plastics/aluminum/brass, etc. No steel milling. Part of the reason for using aluminum extrusion for the top of my machine, other then the two points i mentioned already, is that they can provide quick easy channels for using holdowns for the piece to be milled. (my machine I'm also designing for vacuum hold down as well, just need to come up with the money for the high volume pump needed).
Rack and Pinion is not more accurate than precision ground ball-screws, I agree, however, for a large machine (like a 10 or 12 foot), then I think Rack and Pinion is the way to go for a machine that would mostly be used for routing wood, plastic, brass, aluminum, etc. If milling steel, then you may want to step up to a ball screw and get a larger diameter for the long length to help reduce the 'whip' the ball-screw may have. Another advantage to using rack&pinion is that they can be used for fast jogging/routing without much backlash in them (if any if adjusted correctly).