A backup software with its configuration catalog distributed on different hosts may have scalability issues due to the problem of keeping all this pieces of information up-to-date.
The other things to consider are:
If you lose one of these hosts, a part of the configuration is lost.
You need to protect different pieces of catalog on different hosts, this makes the implementation of a DR solution for the backup environment more complex
NetBackup approach is to store all configuration information on a single host, the Master Server, obviously this host will be protected with the highest level of attention and requirements also using Symantec Storage Foundation and High Availiability solutions.
The NetBackup Media Servers are simply there to move the data from the clients to the storage devices, if you lose one of these you certainly lose horsepower but you don't lose piece of information that can compromise the functionality of your entire backup environment.
This NetBackup approach is proven and mature with 20+ years of experience in the largest infrastructures.
Also NetBackup V-Ray technology has performance and scalability as first objectives, the customers can implement an off-host SAN based backup solutions that off-load the backup activity from the ESX servers. VMware Intelligent Policies (VIP) simplifies the policy management for environments with thousands of VMs populating the backup policies in a dynamic way based on pre-defined criteria, this feature is very appreciated by backup administrators that are managing day-by-day backup operations on virtual environments.
That's why NetBackup customers are using the same approach to protect large virtual environments with thousands of VMs toghether with physical servers.
Enterprise environments are often heterogeneous, the NetBackup OpenStorage Program allows customers to protect their investments on third part backup appliances with a better usage of their functionalities. Customers can also opt for a more integrated solution based on NetBackup N5220 Appliances where Media Server and backup appliance are deployed together in a single solution.
What about the management of the overall infrastructure? With NetBackup you simply need to install and maintain one agent on the servers you want to backup, all the database option are included together with BMR and deduplication options, the type of backup is selected at the policy level.
As Kyle correctly pointed out in one of its blogs "Infinite Scalabilty" comes with infinite resources and is more theory than real world experience.
The NetBackup platform is developed with performance and high scalability with less complexity in mind and has a proven track of records on all of these aspects.