The first reason is gear. All class combos suffer from this. The reason is you need 2 sets of gear. While some class combos might be able to wear leather/leather they still suffer from level restrictions. One combo I tried was K/P and when leveling the Priest secondary I needed to drop my current chain and 2 handed weapon for level 1 cloth and a stick.
While this isn't bad at level 1-10 I can see it being a problem when I want to tank and need to level my secondary Priest at level 45 in cloth with a stick. I want a Pally I don't want to level a cloth class. Why should I have to have 2 sets of gear? Sure some people may want to have heal and tank gear but, some people don't.
You do not need to have 2 sets of gear.
You can play your Knight, and level your priest class through daily quests.
Do the daily quests as a Knight and turn them as a priest.
I hardly switch my classes + gear to level my secondary class.
I feel gimped leveling both the primary and the secondary. I feel I am leveling 2 separate toons. Why be a level 10/10 K/P when I could be a level 16 Pally? I can wear plate and heal without needed to go a a frickin npc to swap classes. This should be an out of combat option.
You have had a taste of RoM and you do not like the way the game is played. That is justifiable.
To be blunt, if you read about the game, it did say about multiclassing so despite your dislike for the game, you did go into this game with both eyes open and that it did differ from the game you are used to.
Sooo my point is I would rather play "1" class that is what it is. A Pally is a Knight/Priest so why do I need to level a knight then a priest? This game wouldn't be so bad if I could split my XP. If I wanted to level Knight more I could split my XP 70/30 or if I wanted to heal more I could level 40/60. I hate going back to the same zone and leveling on the same spiders and bats.
The only up side is waiting to turn in all quests in after swapping classes then leveling your secondary with quest XP but, I see this option going away or slowing down at higher levels.
Based on current game setup, you cannot have both ways.
You can either have 2+ classes that have 2 sets of different gear or similar gear, and level each class as seperate entities or do the dailies.
Hope you stay, it is a good game if you are willing to stay a bit longer to enjoy the guilds, the quests, the players, and the monsters.
RoM has it own merits in itself that millions have found on their own.