It may slow down the system, but there is no way around that until Creative releases a ExpressCard audio card. All USB consumer sound cards use software mixing, but at least with the Creative ones the CPU cycles produce EAX 3.0 effects in games. If you only want the EAX effects for games, getting the Audigy software upgrade is more convenient than carrying an external device.
The reason to get it is because it adds support for positional audio. So, if a game supports DirectSound3D or OpenAL, the driver will be able to place the sound source in a virtual surround field. This creates a much more "natural" sound, especially with headphones. For me, games sound A LOT better with this headphone virtualization (called CMSS by Creative, just make sure to select Headphones in the Speaker Configuration if you use headphones). The same applies to speakers, but the improvement is less pronounced.
After all that, I don't think it'd worth $30, it should be free with the computer, just like EAX 2.0 support was free with the SoundMax in my old Compaq 700us.