Well you know how the update "carrot" works...they include a feature or two that you just can't live without...thus encouraging you to update.
Could that "new" 1 or 2 features be included in the previous update...maybe...but then they wouldn't get the extra $49 bucks out of users with the new update!
In this case maybe the version 5 update is worth it if it truly has features that improve reliability with Windows 7.
BUT...if the next Parallels Update (version 6) contains more Windows 7 improvements...then I could see users being upset...since those improvements could have possibly been included in version 5!
- Nick