Quote Originally Posted by Ultimatecalibur View Post
First crossclass slot opens at level 1 or 5 (can't remember which) and the second a 10.
FB->SB->SS and HS->SS->SB are 150 potency/350 enmity for ~63.333 TP per gcd vs FB-SB and HS->SS 175 potency/375 for 65TP per gcd. This is similar to the difference between a LNC using TT->VT (175 potency for 65 TP per gcd) and spamming Impulse Drive (180 potency for 70 TP per gcd). It's a slight TP advantage at the cost of damage/enmity generation and there are exceedingly few fights where it would have a noticeable effect, but the effect still exists. The reason exists but it is far from a great reason.
FB->SB->SB is identical to FB->SB->SS, there is no difference whatsoever. It is entirely redundant for MRD/WAR to get SB and GLD/PLD to get SS. The point of allowing SB & SS to be cross classed is so that PGL/MNK or LNC/DRG have access to a high threat move in case they wanted to act as makeshift tanks. Say your tank DC'd or something like that or you're doing a FATE with friends. You can just SB/SS spam as either class/job and generate 300 enmity per GCD which is the best pure enmity generator those 2 classes/jobs have access to (although you have to give up a LOT of DPS to get it). SB/SS is just given to WAR/PLD respectively as a matter of completeness because they get ALL of the cross class abilities from the other class, whether they have any purpose or not.

It's the exact same situation between SCH & WHM with Physick/Cure. They're basically the exact same spell in terms of cost and potency, so there's no reason to cross class it whatsoever. It's just included as a matter of completeness.