If the two other
![green-dra green-dra](./images/smilies/green.gif)
where already discarded, I'll definitively discard
![green-dra green-dra](./images/smilies/green.gif)
.
In this case, the dragon pair could not be improved and would forbids tanyao.
But if they are still in play, yakuhai is worth just as much as tanyao and is more flexible (use only one set VS the whole hand).
By discarding
![green-dra green-dra](./images/smilies/green.gif)
instead of
![3-dot 3-dot](./images/smilies/3d.gif)
you're moving away from tenpai, so it should provide some compensation.
Here, I don't see much improvement.