You want to have a focused build but u want to have the opportunity to use different stuff fx especially in Dark Souls you want to be able to cast magic but lets face it with a limited amount of casts going full casting is probably not the best idea, so you focus your build on magic and make your weapon do magic damage which scales with magic stat and so forth but even as a caster you want endurance to roll and maybe even to wear some heavy armor once in a said:
at least some beginner miracles like heal come in handy now and then when the Estus flask is empty. But on my first playthrough I'm trying a lot of stuff as well. I heard min-maxing is the most effective way of playing this game (and demon's souls). This game encourages jacks of all trades, I think.
In my opinion, you never should gimp yourself by sticking to one playstyle. I don't know specifically about the ghosts, but to cast magic all you need is to put a couple points in attunement and buy a spell. Well, classes are meaningless after the very beginning.
