By firmware upgrade I am guessing you are refering to a Junos upgrade.
There are only 3 reasons I would think of recommending an upgarde:
1. There is a known bug with a current version that you are experiancing issues with that will be fixed in a later version.
2. There is a known seurity flaw with a current version that will be fixed in a later version.
3. There are features you wish to use that are only supported by a later version.
Your business may have other policies such as ensureing you are within EOL (End Of Life) of your current version. This means JTAC will investigate any issues (so long as you have a support contract). If you approach them with an issue for a version that is outside EOL then the will almost certainly recommend a Junos upgrade.
Whether in a VC or not, an upgrade requires a reboot which will cause an outage on any ports associated with that member.