I would argue though that the instructor should be grading students in both arts... Rather than trying to blend Hapkido in to Taekwondo and just consider it an enhancement, why not actually learn both arts.
I know that's common Korea and I know at least one member of this board that holds high ranks in both, so why use it as justification to pad the Taekwondo curriculum - why not just do both arts and grade separately?
Having seen the school where I train shift from a hybrid TKD/HKD program to offering two straight TKD and HKD classes, I can find arguments for both methods.
The hybrid class only ranked in TKD, which was fine. Nobody complained and everyone enjoyed the change of pace on 'Self defense nights.' It also made scheduling easier.
With two seprate classes, most students will try both briefly and end up gravitating to one or the other. I learned the hybrid class and tested for my ildan shortly before he split the programs off. What I observed is that pretty much all of the adults gravitated to HKD and very few of the kids, while a few adult stayed in TKD, but the vast majority were kids. The adult TKD class pretty much vanished entirely.
Another factor is that movement in hapkido is different than that of taekwondo. Not better or worse, but different. While the hoshinsul can be adapted to taekwondo, learning hapkido as a martial art as opposed to taekwondo with hoshinsul culled
from hapkido were very different. Again, not a better/worse type of different; just different.
Teaching the two as separate arts has the benefit of people being able to experience something very different from taekwondo and enables the school to reach a wider number of people.
Teaching them together, however, makes one art more appealing to a broader group and enables you to maintain a more streamlined schedule.
Lastly, there are a good number of TKD instructors who have picked up some hoshinsul culled from HKD but who are not really qualified to teach hapkido as a separate course of study. They have worked these techniques into their TKD curriculum nicely and they need not bother with ranking in two separate arts.
Daniel