- Reducing costs - as Brian Madden pointed out, VDI can be much more expensive than Terminal Services. A hybrid approach enables you to match the most cost effective technology to each user.
- Making the entire process whorthwhile - in some cases if you don't centralize everything, the business value of the processes is decreased significantly. Indeed, in some cases it is literally all or nothing. For example, if regulatory requirements demand that all data be stored in the data-center, having even a single user that still has to keep some data localy is unacceptable.
Ericom has published a whitepaper that highlights the benefits for the hybrid approach to centralized computing. You can find it here.