Fairly to very useful, depending on what you're up to. 4.5" starshell was going out of life (wasn't part of Improved Ammunition) because the assumption was that our troops had enough night-vision kit that starshell wasn't necessary any more (as well as onions' point below about getting visual ID on surface contacts), but that relied on the notion that it was only being fired for our eyes.
Enough starshell got fired during ELLAMY that they were combing through all the leftover holdings to keep LIVERPOOL supplied: there were a few times where misbehaving regime forces couldn't be engaged with HE because of targetting and collateral concerns, but having some very bright lights appear in the sky above them persuaded them to go find somewhere else to be naughty. There'd been similar experiences on land in Iraq (at least, got woken up by the 81mm mortars once or twice) where a sudden skyful of white light had a useful "we are watching you" effect on suspicious people, and was useful for allies not so well-equipped as us.
Probably driven by that experience, there's a new buy underway of 4.5" star in white and dark versions (dark only emits in infrared, so it improves visibility for night-vision gear but doesn't help anyone just using eyeballs) to keep the stocks up.