Yes this is a common practice. GroupWise "REQUIRES" eDirectory for management. Without eDirectory, your GroupWise will not be very useful. (There is a technical note here... GroupWise does not require groupwise to run, but you can't manage it without). You cannot manage GroupWise with Active Directory. Not in GW 7, and not in the GW 8 soon to be released (end of 2008). There is speculation about GW9, but it's too premature to go there just yet.
It goes against best practices to put eDirectory on an Active Directory domain controller. eDirectory can be installed on an Active Directory member server with no problem.
As Goombawahoo mentioned, when you purchase GroupWise, you get the Suse Linux server license for free, no extra purchase required. If you put it on Windows, you will need to work out licensing for Windows Server or any other access component. It only makes sense to put it on Linux since Linux overall is more stable and performance is better.
For reference, since it sounds like you are not clear on what Linux can do for you... Open Enterprise Server (OES) is the next generation of NetWare. Yes it is Linux, but it provides the same services as what NetWare does. In fact, you can do a migration to OES and it can be done completely transparent to the end users. We do this all the time. So you don't have to reinvent the wheel like you will if you take things to Windows. All your printing and file systems, including permissions, printer deployment, etc can stay pretty much the same overall.
