items in play vs keeping them in your hand

[Clipper]

Your points are all valid except the last one about discarding to fuel a power. If the power does not mention a source of where the discards must come from, then you can discard from the table or your hand. For example, the Wizard's Charm Spell says you must discard your whole hand, so it has a source, but the Wizard's Flight Spell does not provide a source and you can choose to discard from either your hand or the table.

Another reason you might want to play these cards to the table is to avoid curses. Curse like Curse! Income Tax, Curse! Lose 1 Small Item and Curse! Cursed Thingy are all pretty weak when you can just choose one of those Items you can't use.

I personally keep them in my hand until I need to avoid Charity or want to trade with someone who could use them, though. I like the element of surprise that can generate.