I said harmful becoz, you will install any kind of software. Many of these unsigned app are moded by hackers & crackers, so may there hidden suprise for you it doesn't mean you know unsigned app & you are safe.
Install unsigned apps only if you are sure of what you're installing and you completely trust the source from where the software comes from.
There are people with enough knowledge about mobiles and symbian, who wouldn't mind installing unsigned apps and know their way around. But if you aren't sure it's better to keep away.