Softimage XSI has recently been acquired by Autodesk, the company that owns competing packages 3ds Max and Maya. Users who've purchased XSI through its former owner will now have to deal with Autodesk for any licensing and support issues.
Since XSI requires the software to be activated online before it can be used, existing users will be at Autodesk's mercy when reinstalling XSI onto their computers following catastrophic data loss or when transferring their license onto a new computer. These users, who purchased XSI with Softimage's particular activation policies in mind, will now be subject to Autodesk's activation policies and may even be required -- if Autodesk should so desire -- to agree to Autodesk's more restrictive licensing terms before Autodesk will agree to activate any copies of XSI purchased before the change of ownership.
Perhaps things will turn out okay for current XSI users, but there's no guarantee that will be the case. Whenever you use software that requires activation, you remain at the mercy of the software's copyright holder, which may change without warning and without any remedy to you, the user.