I understand what you guys are saying, and I agree that this seems to be an odd choice.
I can only imagine they're doing it in order to have an interaction between Dorian and Marius, either to build a relation between them or simply to draw out a discussion about Tevinter or Marius' past. Probably both.
Frankly I kinda wish they had just stayed in Tevinter and introduced new characters, but maybe they were worried they would have trouble attracting people unless they got some established character cameos in early.
Actually, since you bring this up... the following story arcs are going to take place post-game. Since we know that Dorian returns to Tevinter at some point before Trespasser, and that the Venatori are still active there, we might indeed see Dorian further along as well.
The hitch is that, if you ask, he stays in the South for most of the two years between the game and Trespasser, but if you take the "I understand" path he leaves a bit earlier. We also don't know how far along this comic is going to go post-game, whether it will intersect with Trespasser, or end right before Trespasser takes place and the Inquisition is folded into the Chantry or disbanded.
Regarding your comment, I can now see the comic timeline extending beyond Trespasser with them heading back to Tevinter and possibly joining up with both Calpernia (whose death is NOT confirmed in the game) and Dorian in opposition to the Venatori and their goals. Since this issue will have already established a meeting between Dorian and our duo, the stage is set for that relationship, which will possibly extend into DA4.
Also something to consider about the inclusion of the Chargers. This means one of two possible scenarios.
1 This is mid-game and the Inquisitor has not gotten the Qun mission from Bull yet.
2 The Inquisitor has gotten the mission and saved the Chargers.
If 2 is true, then in this world state Bull is now Tal-Vashoth and will not betray the Inquisitor (or Dorian) in Trespasser.