It would be very helpful if you could assign a client to multiple client groups. As it is now, you can only assign them to one group. However, the ability to assign them to multiple groups, opens up a whole level of functionality to module development.
I have some modules, such as dashboards, that display detailed information about a client's services or relationship with my company. I use a client group to give the proper access to clients accounts (multiple client accounts for the same company need access). Also, you use client groups in the mass mail tool. I categorize my clients, in client groups, based on their main product/service interests. Even if someone signs up, and hasn't bought anything yet, they get assigned to a client group. This is very useful when using the mass mail tool.
Even though I don't use this feature, I know you can give discount percentages to the client groups (Group Discount). For multiple groups assigned to a client, I can see this possibly using the highest discount of all groups the client is assigned to. Another words, if the client is assigned to 3 groups and 2 have a 10% discount, but the 3rd has a 15% discount, they would get the 15% discount.