I also would like to see that.
I've got lots of PBS storages pointing to different PBS and namespaces. Half of them are read-only so useless to have them in the list if backup targets.
And of the other half, 2/3 of them (namespaces for weekly and daily backup jobs) are always disabled and will only be temporarily enabled by VZDump hook scripts. I only care about the last 1/6th that point to namespaces used for manually triggered backups. And half of those are only available 1 day per week while still being enabled. Unluckily those enabled but not available PBS storages are first in alphanumerical order so whenever I open the backup tab it will hang for a short time trying to pull data of the storage that isn't available and I then have to wait to select the available one as the backup target. Would be really nice if the webUI could remember the last-used storage and pre-select it.
And yes...I could disable that storage after each manual backup and enable it again before doing the next one. But I'm too lazy for that and I can't work with VZdump hook scripts as for that I would need to start the manual backup and I can't start it when the storage is disabled as it then won't be shown in the backup storage selection.