I think you need to have the required lvl to wear a portent. It’s not fair people can Buy Runescape Gold and use portent of life while being 10 divination. Because then there is no need to lvl up divination.
Attuned portents of restoration are untradeable, while the tradable regular versions are tradable but only heal 75% compared to the attuned version, so those who want the most out of their portents would need to train themselves.
Portent of life isn’t tradable. Also portents aren’t worn, they’re the things you just keep in the inventory, it’s signs that are wearable. The sign of life is tradable, but is goes in the pocket slot so gets in the way of people using scrimshaws and other potentially useful pocket slot items.
Not to mention people who train divination and make their own don’t need to pay whatever price it is the portents/signs are going for. (I don’t know if it’s actually profitable for people to make them compared to just selling the component materials outright, but still.) People don’t need cooking levels to eat cooked food, or smithing levels to wear armour, so it’s just ways that divination could make people Runescape Gold on the side if the market happened to swing that way.
Also, even if people don’t immediately recognise the need for divination now, people will inevitably be grateful to have a high divination when skill 2 comes out, as energy will be used with it.
edit: I almost forgot to mention one of the best things to come from divination, and that’s the signs of the porter which aren’t tradable. Automatic resource banking is sweet.