If you're going to create an application performance profile or baseline, you need the GuestOS specifics in many situations. You need the OS information in addition to application-process information (java, sql, etc), to name a couple.
The advantage to having Guest-level metrics is that to you have more visibility in to your workload. Without, you're just observing the virtual machine behavior - which can be satisfactory or unsatisfactory depending on the type of workload you're trying to profile.
I usually see Hyperic and SCOM. They both get you guest-level stats, but Hyperic integrates a little better with the Web Client, agent deployment/management, suite bundle licensing, and vCOps features OOTB (native relationships creation, etc).