In Golden Sun when the Open Helm can be first bought in Bilibin, it is a pretty good upgrade to the Leather Caps that Isaac and Garet are probably wearing, and is worth your generally low amounts of funds you would most likely have at that point. They're much better worth your money than the Wooden Cap, which has a single negligible extra defense point over the Open Helm but costs over twice as much (400), so it would only make sense from a financial perspective to only buy open helms for Isaac and Garet.