Having a "main" class is normal--you will probably find a class or two that you really
like. However if you don't try out the other classes and level them at least a little it can hurt the very class that you DO like. I'm not saying you have to keep all your classes at the same level--but playing the others at least a little can really help.
Your CLASS level cannot exceed your CHARACTER level. That means you're slowing yourself down on leveling the class that you like if you stick with the same one.
Certificates come in all shapes in sizes. From (EVA +4%) to (MP Cost -2%) each class has it's own set of certificates for you to use. These certificates can be used on other classes. That means I can put a Mage's ( Malice -5%) on my Engineer so that I don't pull aggro if I want to. Furthermore, there is a "Yellow Stone" between all the certificates that changes based on the certificates around it. You can get new "Yellow Stone" bonuses by having more kinds of certificates.
NOTE: You need 2 or more of the same colored certificates to activate the yellow stone. It will be dimmed and not active otherwise.
You can access your Expertise score by pressing "K" and selecting the "Class Roster" tab. It is located in the bottom left.
Expertise level is based on "Class Score."
Class Score is based on each classes's level. Since it is easier to level at lower levels, this can bring your "Class Score" up faster.
Here is why that matters:
Expertise is basically a set of Bonus Stats.
Class Expertise Level 1
Max HP and MP +75, ATK & M-ATK +84, G-Healing +9
Class Expertise Level 2
Max HP and MP +75, ATK & M-ATK +101, G-Healing +11
Class Expertise Level 3
Max HP and MP +136, ATK & M-ATK +181, G-Healing +20
The list goes on--it's very useful so check it out. Make it your goal to get to the next Class Expertise level!
Never get stuck calling for a certain class ever again! If you need a tank and all you see are healers, you can swap to your tank!
Also, some bosses are more susceptible to certain classes. For example, on bosses with hard hitting AOEs, having both a knight in and a Martial Artist in would not be a good idea. Instead, the Martial artist could swap to a ranged DPS for the boss-- and back to their hard hitting martial artist for mobs without those difficult AOEs.
Certain Skills are extremely useful even if you are not heavily focusing on that class:
(This is just some.)
Spatial Conversion: Teleports any members of your party who are on the same map as you to the last soul guardian you marked at. This can really save you time especially on the larger maps.
Hard Truth (MP+HP Buff 15 minutes), Grace of Wisdom (INT+, WIS+ 15 minutes), Light Blessing (Luck, Nature & Holy Resist 15 minutes), Revive (Reduces EXP loss vs reviving to town/saves you from running all the way back through a dung)
Mind Surge (Restores MP, can be used between pulls to speed things up)
Several 15 minute buffs...sorry don't have all the info on them. P:
Poem of Wind (Swap to your bard, buff yourself and GO! Increases Movement Speed 10% 15 minutes)
Sneak (Get to bosses safely)
A great way to get to play your favorite classes is to pick TWO main classes. For example, I play mostly Knight and Engineer--I play the other classes when I am doing guild quests or easier quests because they catch up faster and I don't need to be quite so uber outside of the dungeons. So yes--I do play ALL of the classes. It's not too hard and it helps me understand better how to play. I understand the tank's role better and why it's important that I dispel constantly or stun constantly. I understand to wait for my cleric to get MP. Now go play! >:3
TLDR; Play different classes, it helps.