So logic had nothing to do with it. You know I don't mind re-occurring characters if the story is a continuation (like ME), but since DA games have basically been stand alone as far as the PC is concerned I do wish BioWare would come up with some new characters instead of recycling old ones ... or worse yet, forcing them to fit where it makes no sense just for a cameo (looking at you DA2).
What logic? Are you implying that you already know what the story is going to be about, when you claim that he doesn't suit this story? The difference with Varric to having say... Alistair as a party member again is that there was never enough thorough examinations of his background, only vague hints to his past which are never explored -- He spend too much time talking about anyone but himself so we hardly knew him as much as we thought in DA2.
Let me phrase that I somewhat share your views on having old characters constantly showing up again like a distant relative knocking at your door unnannounced, proceeding to wreck your place then immediately leave. I don't believe, however, that applies to Varric as there's more to him than just being a smarmy dwarf who doesn't seem to have many flaws (or perhaps they were never explored as much).
If you want in-game explanations then I'd argue his experiences in that pile of brown & gritty gray noise that was Kirkwall makes him valueable to have along. I'd say that's enough, really.