1. The Korg is one of the better arrangers out there as you can at least load in 3rd party sounds, (Akai) however an open keyboard can also load VSTi which takes it way beyond anything a closed arranger can achieve. (There are hundreds of different VSTi manufactures out there)

2. You can fully update the OS to achieve whatever you want. (A closed keyboard has limited resources to do this (Lack of available ROM memory etc.) so that if you want a big upgrade, you have to buy a new board)
EG. To update to a T3 from a T1, or a PA1x to a PA2x you have to buy a new keyboard, however if they were open (Software) you could have just bought a software update to do the same thing at a fraction of the price.

3. New software often requires updated hardware, and closed keyboard hardware cannot be upgraded without buying a new keyboard, whereas an open keyboard uses a standard PC computer to run the software, and so can easily be updated without needing to change the entire keyboard.

4. All open keyboards have at least a 24 bit 96 KHz sound system, (The studio and production standard for many years) whereas closed arrangers tend to have the old 16 bit 44 KHz system. (Now coming on for 30 years old)

I hope the above helps to clear up some of the confusion

Bill