Russ,
This is just a best practice/recommendation. Since ucsCentral is a management application we're recommending to host within the scope of your other management devices. We have many customers who don't run a separate/segregated management domain and who chose to run ucsCentral on the UCS domains it manages. There's no probem with this technically. It's a similar recommendation to running your vCenter on an ESX host it manages. Personally I have no issues with this myself and most customers deploy this way, but in a best-practice world design suggests to keep it separated.
Regards,
Robert