So the question to be asking here is:
Do you want the features RoboHelp Server offers?
I know John has responded in other threads where he has outlined the features in more detail, but I gave up searching after finding the thread I linked above. Feel free to search for forum posts by John Daigle and you should unearth such threads.
RoboHelp Server does offer some interesting features. I'm sure John will pop in and fill in any gaps I missed.
Helpful and Handy Links
Thanks to Rick for including the link to my post on the benefits of RoboHelp Server. To your point, I would add that the old RoboHelp Server for .NET was created years ago by eHelp Corporation and Adobe judiciously dropped that from the lineup when they took over RoboHelp from Macromedia.
The nice thing is, they now include full .NET and C# API - for free! You'll find it along this path in your RH installation:
C:\Program Files (x86)\Adobe\Adobe Technical Communication Suite 2\Adobe RoboHelp 8\CSH API
So, you would not need to purchase RoboHelp Server, IF .NET was the only reason.
Instead, the benefits of the RoboHelp Server Feedback Reports and being able to create audience areas (like websites of their own) with authenticated access is very valuable to many folks and that should be the real reason for choosing the RH Server option. I hope you take a closer look at it.
Adobe Certified RoboHelp and Captivate Instructor