Suppose you own a 2007 model of the then-cutting-edge, high-end laptop of a given brand A with say 4 GB of Ram, an 2.4 GHz Intel Core 2 Duo. Further suppose you want to purchase the current display offering of the same brand (available since Oct 2008), wouldn't you expect to be able connecting both expensive pieces of hardware without yet further investment, say without purchasing a converter that's not even available from brand A? Of course, I should get a 2010 model of an A-branded laptop, and everything works fine until I guess 2011, when they replace the Mini DisplayPort by say a new "Nano DVI port".
Suppose you are very happy with the display, the computing power, the browser speed of your lovely smartphone, again from the same brand A. Occasionally, you would like to write some longer emails or edit online documents, or ride on a wave, for which it would make terribly sense to have an external keyboard. Technically, it is trivial to enable this through bluetooth, unless brand A decides to cripple the bluetooth driver specifically, in which case you can always buy the new iPad-like device of brand A, for which a keyboard is available (unless you are living in Europe, where you can't purchase that device yet).