Just a tid bit cause I been down your same road. As I just finished starting up a hosting company solely around DNN. Read up on your SSL / ip conflicts as those are the only main issue.
DNN in nature takes very little resources to run decent, but does scale up a bit based on the amount of users, but you tend to find its a bell curve. Mainly cause of how asp.net works with its worker processes.
Per the above reasons, I split seperate clients out into their own installations and bite the bullet and get developer / enterprise licenses when I get 3rd party modules. But have been very thankful for doing so. As I started to max one server out cause a client started becoming a huge resource hog and kept wanting to expand, all I had to do was setup another account and move the db's / file systems over poof they left my one installation and moved to the new one, no one including the client knew it happened. Was about a 5 minute rollback of the site.
HOWEVER I do have several clients which are using parent portals. For example the local sports group they have one install and 6 parent portals. The main site gives the general overview of the group, and the 6 portals are dedicated to each region, North, South, East, West, Central. Each of the sub portals has its own skins, content, layout, branding, etc.. There should never be a need to move one of the sub portals out or away from the main, as they are all tied together in purpose.
An example would be like ESPN. They wouldn't want a seperate install for each sport type. IE football, baseball, etc. They would want a "common" login for the visitors, but might want the option to www.espnfootball.com be a parent portal.
As stated above though its much more about how / who is using it and what they want to achive. DNN just gives you the tools to implement them anyways you want. WEll to some degree.