Hi,
Generally clients choose dedicated servers as they need more flexibility or to install their own applications etc, recently we have contacted a few customers who's sites were using too many server resources and have now moved to dedicated hosting.
You can't keep adding resources to shared hosting as you say, but a dedicated server will give alot more flexibility and the ability to host many more sites as well
If you wished to move to dedicated hosting we can assist in moving any existing sites and setting the server up to your requirements. If you wish to discuss further please contact me -
paul@catalyst2.com