That's interesting. I do pretty much the opposite. I don't think this is a case of right vs wrong but more of a prefernce as to which skill tree you prefer to use as your primary.I prefer to use cold facets -the fire will be strong enough as it is. My sorc has all cold skillers plus cold facets in the orb/helm/armor/ +2 in my Moser's.
Personally, I run with 5/5 fire facets in my orb/helm/armor. I use a Ber'd Stormshield (hardcore). I prefer to use fireball and will do so everytime except when I meet a fire immune (which seems rare given I run conviction from my merc).
This brings me to why I prefer Fireball over Orb as my primary skill. If you are using an Infinity carrying merc, a fair amount of the fire immunes out there will have their immunities broken such that your Fireball will still be effective. I'm rolling with -30% resistance from Tals/facets so once that immunity is broken they get shredded. Lister's pack is the noticable exception as they remain fire immune but a nice Glacial spike followed by a couple Orbs.. (rinse and repeat) while your merc ownz them all and its pretty safe I've found.
Many of the cold immunes though seem to have really high immunites such that your merc's conviction won't break them. And for me, most of the real annoying monsters tend to be cold immune such as Oblivian Knights and blood star throwing Harpies. So I jack up the power of Fireball and skate through most areas of the game.
Just offering another perspective.