One of my clients has a 5 year old entry level Acer Travelmate that he uses for everything from document editing to CAD work. It's a bit slow in rendering but still works 100%. Only difference you'll see with a higher specced laptop is everything happening a bit (to alot) faster... So your argument does not make sense to me.
Depending on what type of CPU you are using. For instance: Would Autodesk Inventor 2009 run on a 5 year old notebook with most likely a Celeron or P4 CPU, compared to a new Notebook computer with something like a new Celeron or C2D CPU...?
Eg. 5-year old notebook's possibly doesn't have a display system which will render 3D objects whilst drawing a 3D mechanical drawing, so you will be left with a slower and less productive working environment, than using a newer higher spec notebook...? Hope this makes sense...
It all depends on the user's requirements. If you want to do high-end multi-media processing (including CAD, database development, sound-engineering, etc.) do you still want to buy a R4K notebook...? AGAIN - Spend the money on something suitable for you. You will get frustraded with something lower-specced if you actually require a higher-specced system...
Hope this states my point (even though its only a personal opinion) as if you go cheap on a PC / Notebook, you will mostly get what you pay for... Compare it to a car - If you buy a R50k Citi, do you expect it to perform like a R300k GTI...? Stupid comparison, but anyways...