One of the issues I see with a SoftPLC controller... Comm bottlenecks.
The ladder logic talks to everything as Remote I/O...everything is sitting in a remote rack. The SoftController has to behave like the CPU and an H2-ERM module. Now imagine, If you had an ECOM100 in your base to communicate with a drive network... it's a double hop.... Whereas....an H2-DM1E is just going to communicate with the ECOM100 over the backplane. If you had an H2-SERIO module, it would be sitting in your 'remote rack' too.
Next, cost. The overhead associated with deploying a PC as opposed to sticking a $399 H2-DM1E CPU in place of the $301 H2-EBC100 in your base, I just don't see any cost advantage at all...
Lastly, reliability. Bob couldn't have said it any better. One of my major clients has a ton of Proprietary Gauging Systems (Infared, Beta/Gamma, etc), Drive Systems and Scales that they need to monitor/control and archive data. OPC Servers are used to retrieve data from each of these systems and ultimately marshall it for display on WonderWare HMI and archiving to MS SQLServer. Because of the Network Topology and Equipment/Factory layout, it is almost always necessary to install the OPC Server PC very close to the devices they are extracting/writing data to/from. These PCs are one of the greatest single point of failure for this client....dirt/heat/moisture kills these things. The Drives, PLCs, Controllers, other industrial equipment, etc... all continue to run reliably.
Nineteen years of supporting various clients with PC interfaces (serial,opc,other proprietary) to their Industrial Control Systems (DCS, SCADA, PLC, etc..) has cured me of any desire to do real-time control on a PC.