Can't you already wear all the chronocharms at once?
Anyway, think about it like this: At most levels (depending on your DM), you'll probably have a small number of magic items. You don't generally have crowding issues until the higher levels, at which point price is less of an issue than body slots. And even then, you're paying 50% more, which is a very significant amount, since any item worth adding onto another is probably expensive.
Let's say you're a Druid and you got 20,000 gold in a good dungeon crawl. You've already got a magical belt, so you consider adding the effects of the Monk's Belt to increase your AC. If you just buy a Monk's Belt to replace your current one, you'll spend 13,000 and have 7,000 left over, for expendables or spell components or an Amulet of Health +2. If you want to just tack the effect on, you'll spend 19500, leaving you with 500 gp to spare. So it's a pretty significant investment to add an effect to an item.