The advantage of a Soekris or similar solution is that it is purpose-built. The hardware is usually very stable and oriented at its exact role. Your netbook could not perform any of the tasks for which Soekrises are generally intended:
- Wifi AP
- PBX Server
- Light-use file server
The first three are not options because you only have a single NIC. The last three because you have no PCI expansion capabilities (for AP-capable wifi card, PRI/phone line termination, and RAID card, respectively). USB ethernet/wifi adapters are simply not feasible replacements for built-in capabilities.
A built-in monitor/keyboard is really not much of an advantage. Most embedded systems pump console ouptut out their serial port - you don't want to have to worry about a screen/keyboard/trackpad/etc in your rack or under the stack of hardware.
The built-in UPS is an interesting idea, but not compelling enough to overcome the other glaring deficiencies in a netbook.
Netbooks and laptops are best used for their original intent - portable end-user computing. Possibly there are a few minor niches where this might be useful (hobbyists who just need something running a particular OS permanently connected to their network), but I can't think of any within the scope of this forum (IT Pro/Sysadmin).