Don't hate me, but if you like playing the archer more than the mage, you should create a new character that is an archer as the main and play that character.
There are some features of the game that only benefit your main, not your dual. For example, if you were to complete the level 107 Exalted quest on your dual, but your main is only 105, you won't be able to unlock the Spellbook: Dignity of the Exalted. You'll have to level your main (the mage) to 107 first.
And when you pick your Dual Certification Skills, it makes things easier if you are picking skills for your archer, as opposed to splitting up the skills to help both classes a little. Say for example, if you are the type that was considering Dual Certification - Physical, or Dual Certification - Magical. Which would you choose? (I'm not saying that these are the Dual Certification skills that you'll want, or that they're the most optimal for either your class or your style of gameplay.)
And then there's the predicament that some quest rewards cater to your class. Sure, you can swap Exalted equipment. But that will cost you 100,000,000 adena. Even if you're rich, why incur an avoidable expense?
Also, I'm not sure if they've changed the Olympiad yet, but that used to be for main class only.
Further, on rare occasion NCSoft will drop an event that caters to main class. It's been a while since that last happened, but never count against it.
I could go on, but my point is, if you really see yourself playing Yul more than any other class, make it official. Make Yul your main.