They are relevant, you have really high power spells, both when you want to damage enemies or heal friends. When you don't have a horse you are just more vulnerable cause you are very slow and with low stamina so you will not run away from your problems. You either solve them or succumb to them, no other option available. If this is ok for you, Fat Mages are super cool and fun to play.
If you play solo... well, there are easier build to play solo, it's not impossible to play a solo Fat Mage but is definitely not suggested.