You don't really need trainers, do you? I almost never train most of my major skills when I play, I just level them up manually.
There's no way you can justify your character going to Skingrad or Anvil before taking off on the main quest? Maybe they got bad directions to Chorrol, got lost, and wound up in Skingrad or something?
Oh no, I don't -need- trainers. It's purely from an RP standpoint.
I've decided on my main 7 skills: Blade, Heavy Armor, Conjuration, Mysticism, Destruction, Alteration, Sneak - based upon the fact that these skills are what best defines the character that I've written and want to play. But for gameplay purposes, I love having Marksman ability too. From a gameplay standpoint, I do understand that I can just pick up a bow and arrow and use it right alongside my major skills, but from a roleplay perspective, my character wouldn't really know how to use a bow and arrow. But if he were to be taught, then he'd be competent.
Like I said, it's purely from a roleplaying point of view, as there is no in game reason for me to "need" to be trained in order to use a skill.