The reason for such separate development is because, say that your current desktop is optimized for computers with a certain screen resolution (HD) then since most of the netbooks don't have HD or highest HD resolutions thus if they install the desktop then they'll have issues such as applications not "fitting" in their screen, etc.
Another benefit of designing a dedicated desktop is the ability to cut-out some of the unnecessary resources (smaller windows, etc) and optimizing it for certain CPU types, etc.
|Image via: KDE.org|
So, if you have a nebook with a smaller resolution and size then although you can use the standard Qt/KDE desktop edition but as said if you do so not only you'll run into trouble trying to run apps and see their whole windows but also since that desktop is optimized for the usual Desktop/Laptop PCs, etc, so it'll put unnecessary strain on your Netbook thus resulting in a shorter battery life as well.
In that sense if you're a KDE geek ;-) who's not ready to let go of it without a fight, then your best solution is to install the beautiful, KDE plasma Netbook edition.
Another key thing about the KDE plasma edition is that it does not come with a lot of applications which also helps to save some of that HDD (may not be a huge concern, unless you have a faster but costly and smaller SSD) space as well.
But that being said, the KDE netbook version do come with a file manager, web browser, office suite, image viewer, painting application, excellent widgets support and many other applications by default nonetheless + you can easily install many more of other apps in the future as well.
So if you use Ubuntu 11.04 Natty Narwhal, 10.10 OR
since you have a somewhat "restricted" hardware features and hard disk space, you can install the KDE plasma netbook by even without having to install any graphical versions of the Ubuntu Linux. For that you'll have to first install the Ubuntu using the Minimal CD.
And then use the below command in your Terminal window (as said if you've already installed Ubuntu 11.04 using the standard LiveCD, then you can also use the below commands too).
sudo apt-get install kde-plasma-netbook
That's about it mate! :D.