I have a PIX 515E running 6.3.5
Are there any reasons why I would need to upgrade to version 7 apart from keeping the software up to date? I don't need to utilise any of the new features that come with version 7.
if your organisation need is to keep the os updated,then u will have to go for upgrade.
otherwise,as u said,you do not need to utilise any of the new features introduced in 7.x,i would recommend not to upgrade to 7.X until we have a gd release in 7.X or 8.X train.
in 7.x,we see a lot of issues and 6.3.5 faired well above the 7.x as far as stability is concerned.
so,if there's no immediate need to upgrade,stay on 6.3.5 in 6.x train,wait for a gd release and afterwards go for an upgrade.
hope this helps!!