I believe I understand what you're asking here.
It can definitely be complementary to "purpose-built, product specific" monitoring tools that a storage admin may have access to. However, there is some overlap here. Depending on the performance tool used, I believe its main value which differentiates it from other products has to do with understanding core things about storage in order to help model or present data in a meaningful way. This can be response times, RAID penalties, relationships, failure scenarios, etc.
If you are looking at vCOps as a product, it internally as a "solution monitoring" technology does this kind of thing, mainly in analytics, currently focused at the management and hypervisor layers. In solution monitoring what is important just like deep performance tools, is that we understand relationships. With vCOps it understands its solution by a VM living on a hypervisor, which accesses datastores to access its virtual disks.
I believe that the vCOps tool is complimentary as well as a competitor. If the vCOps has the chops internally to setup relationships (it does), do threshold alerts (it does), generate reports (it does). If these are all so then we can start to dig into the benefits of what an analytical engine and a global solutions view comes into play.
At the end of the day where we are driving is towards cloud monitoring solutions which dictates a transactional view. In order to make this sustainable we need something that looks at transactions across any part (global) of a cloud and understands the health of that transaction. It then can look below the covers and uncover abnormalities all the way down to the backend disk. This is what I would dictate the solution monitoring where we are very much relationship minded and doing “top down” analytical review of the dependent stack vs. a bottom up silo and threshold based monitoring strategy.
Real world view of vCOps helping provide storage insights? VMworld 2011 in Las Vegas was the first run with VNX stats being displayed in vCOps. During that show there were performance issues being reported. vCOps was able to identify for us that storage was not the issue and pointed the VMware team to the source of the problem. A quote from the show.. “I previously was driving the Enterprise and asking for more power by yelling to the engine room, ‘more power’! Now with vCOps and bringing all this information together I as the captain am in control.”