You brought up some very good points, Clifford. That was a very nice penny's worth, IMHO.
I, too, started with the Key of C diatonic. And yes, you'll wind up with several diatonics all in different keys.
Although one can play the diatonic chromatically, it takes a master of the instrument to be able to do so, thus, the different keys come into play.
As far as Chromatics go, they are more expensive than the diatonics, and they require maintenace regularly. Because the Chromatic is, well, chromatic (meaning all the notes are there and available to be played without bending), you can play in any key on a Key of C Chromatic. HOWEVER.... there are also different keys of chromatics just like there's different keys of diatonics.
Why the different keys if one can play in all keys on a C chromatic? Because the note layout of the different keys can aid one in playing of particular pieces. Say a song written in the key of Ab (read "A Flat" ) which has four flats in it's key signature, and for whatever reason, you're having a really tough time playing that particular song on your C Chromatic. Well then, try playing the song on a Key of Ab Chromatic and see if you can play the song any easier.
I'm still just a "young-un" when it comes to the length of time I've been playing harps, but I'm beginning to grasp and understand the concept that different keys of chromatics can make playing particulare difficult songs easier to play in one key than another (such as using an Ab harp to play a song written in Ab, rather than struggling to play the Ab song on a C Chromatic).
So, there's my two cent's worth.
By the way, Welcome Roy!