I have keyboards from the HHKB to a 122 key Model F, and I have to say, I find it frustrating using a keyboard without the numpad. I do a lot of programming, and like most people who do that, numbers tend to be confined to variable and constant declarations, all in a block at the top of a file or the top of a function. As such, when typing them in, I find it vastly easier to use the numpad. But when writing actual code, numbers are comparitively rare, since they are all stored in variables instead of hard coded.
Also, I'm more of a keyboard shortcut person. I find that people who prefer tenkeyless generally tend to use the mouse a lot. I have mousekeys enabled in X, and I use keyboard shortcuts for almost everything. Basically, I'm rarely using both the keyboard and the mouse. It's almost always one or the other, and I always move the unused item out of the way. So for me, the arm reach issue isn't an issue.
Whenever I boot up a game like Dragon Age though, I prefer to use my keyboards without tenkeys. Because that's just about the only time I use both keyboard and mouse simultaneously.
So it's all in how much you use your mouse.
