Yeah, that got me at first when I started to play some time back.

But cross role skills CAN be gotten again! You can set them up (you get 5 now as a class with a Soul Crystal, instead of 10 as a base class). But I can assure you, a lot of the WHM (White Mage) class skills/spells/abilities are awesome!

As a WHM (White Mage), you'll at least want (IMO); Esuna (very important!) Swiftcast (helps get those Raises off quicker) Protect (Very helpful if you are in a low level dungeon, though many might disagree. I think it's helpful anytime), Lucid Dreaming (lowers your enmity which is hate, as well as helps regen your mana) ... and the last one is debateable.

I chose Largesse to help with heals, but others like Eye for an Eye.