Erik joins because I thought I could do something interesting with Erik as a playable character. As for his reasoning, I don't want to get into it directly (That's for my editors to tell me is dumb and stupid) but he is only recruited if Hector is alive. It was also me looking at the overreaching theme of FE6 and deciding I wanted one character to represent the difference between Roy and Zephiels Philosophy.
Zephiel thinks that all of humanity is power hungry scum which betrays its loyalty's, family, loved ones, culture etc. all to advance itself. He has plenty of examples of this, primarily starting with Erik, Orun's adviser, Leygance and Devias, Arcard and Roartz, as well as those left to rule the Western Isles, The Djuto tribe and I don't actually consider Sigune this, since Signue's reason for siding with Bern was pragmatic rather than greedy.
Hell his example of this stretches to Narcian, someone who was completely unqualified for his position as a Dragon General, who none the less got it by pointing out that Galle wasn't born in Bern. His only three examples against this are Marduk/Murdock, Brunnya and Guinevere.
Roy on the other hand, believes that anyone can change for the better, and for any reason. Guinevere betrayed Bern out of love for her brother and love for the world, Elffin aided the Western rebels against his own kingdom, Every Enemy Recruit was someone who's beliefs were shaped through coming into contact with Roy and his forces.
It's the argument "What can change the nature of a man?" and I don't feel I'm reaching too much by saying it's apart of FE6's story, it's just never really called into question or brought to life.
I thought with Erik I could show something interesting in how exactly he changes. Because of course, his character across two games is he's a traitor, and even his personal skill is "Scheming" Just because he joins you does not change his underlying character.
So I hope that satisfy's your question.