Please elaborate a bit
Desktop Platforms (I assume you are talking about Hardware)
It depends, if you have a small/medium sized network that might work, but if you have something like a medium+ or large you might not want to do this.
Again, hardware requirments are there for guidance, if you are anticipating rapid growth in your organisation/company network you should consider buying proper server hardware.
Hope this Helps
Depends on desktop platform config: RAM & processor. The drive space/speed may not be as much an issue if the host is only performing DC roles. I would recommend at least mirrored drives for redundancy. And as said before, the desktop platforms may be adequate for a small (<50 user) environment.