OK, first off, the numbered keys and math operation keys you point out are activated as sort of a numerical keypad when you hit the F6 "Num Lock" key (actually, that key toggles the functionality). I wouldn't guess this to have been a frequently-used feature. judging by the availability of USB external numeric keypads, which would have been a more acceptable solution to anyone doing serious numeric entry on a PowerBook.
The "fn" key only modifies a few keys, like the arrow keys to give you the indicated functions, which would correspond to the home, end, and page up/down keys, plus a "clear" function on (I think) the 6 key.
The "enter" key has subtle differences in some applications, like for instance Excel.
Generally, the design issue is the significantly smaller allocated real estate for the keyboard, which required some editing of the normal Apple keyboard configuration.
I see. And now that I've taken a little while to try it out I get the full sense of what a truly stupid idea it is. And the num lock appears to lock out the entire keyboard other than the keypad. I don't know how anybody could get used to it. Personally I bought a USB keypad for my 17".
I tried it a couple of times just for grins in the first month I had the TiBook. I thought it was a pretty lame concept and fortunately never had to do a lot of numeric entry on the book, so it wasn't an actual issue. If I had actually been doing any of that with the TiBook, I would have picked up the fast-selling USB numeric keypad for sure or just hooked up a normal keyboard when I was doing that kind of thing.
BTW, I have replied once or twice to folks who accidentally hit the F6 key and then thought that their keyboard had gone berserk. That was the real problem with the scheme, IMHO.