LiveUpdate - Is it useful?

Absolutely worth the trouble, once you get it set up correctly.  It took me a few weeks to figure out everything I needed to do, including the proper setup of IIS on another of our NBU servers.

The alternative, because you are using WIndows, is you would have to log into 500 clients, copy files over to a temporary directory (just mounting the directory from another server and running the install from there doesn't always work), then run the installer and answer the prompts.

If you installed the NBU client using the "silentclient" batch file, be aware that the LiveUpdate files were likely not installed.  I'm not sure with any versions above 7.5, but LiveUpdate client was not automatically installed with the NBU client when you use "silentclient" batch file in prior NetBackup versions.

My only concern about LiveUpdate is the lack of poss-install cleaning process on the client. The setup files are downloaded to /ProgramData/Symantec/LiveUpdate/Downloads but there is no post-install job to cleanup afterward...

This, plus the fact NetBackup keep all previous binaries on /Veritas/Patch can really eat a large amount of disk space... !

And if you multiply all of that by few hundreds or thousands of clients... 

With a custom install script you can remove all that files.

Fabrice, there should only be 4-6 patches per Netbackup version moving forward.  Therefore the footprint should be minimal, especially if you have logging turned off (best practice) when not required.

Using LU is beneficial when you have a large number of client machines.  Another useful advantage of using LU is when you have performed a Custom install initially on your client machine, those settings will be picked up and maintained.  (i.e. when performing an Exchange backup the NBU services use a Org Mgmt enabled account, these will be maintained).