I'm a jungler with what I'd like to think is pretty good control over both (and stats seem to heavily back this claim up), but I am not a fan of this.
The thing about the Rift Herald is it lasts from 10-20 min mark, but plates fall at 14. Because of this, it has relatively low value after a few short minutes, but within those few minutes has a VERY high value. If you take dragon control you can time the spawn around it, or even value the dragon or herald over eachother and trade objectives without the enemy knowing until it's too late.
All the above said, as a jungler that understands how all the jungle monsters work, I think putting a dragon in place of the herald would be a HUGE step backwards.
Much like the Mountain Drake, the Herald has a pretty long kill time and you have to decide if and when it's worth that time, because for that window, you do open other areas of the map to the enemy. Beyond that, due to its super low "critical high value" period, it should be a highly contested area for that period, and teams should prepare for that.
Preparing means prepping positioning, hitpoints of lanes, and even potentially a sneak dragon. There's a LOT of strategy involved that would honestly disappear around the dragon.
Then you DO get he herald and the enemy KNOWS you want to crack its head on a tower by 14 min. Again, a very short window. Now it's up to the enemy team to keep hard CC pressure on your teammember that picked up the herald for as little as 30 seconds or as much as a few minutes to try to land a channel cancel. A lot of vision, lane control, and positioning is involved here. Or simply to get so much control that the enemy can't get a favorable position to even get a herald on your tower until 14.
You lose almost all of this with a second dragon. Beyond all this, a second dragon would give exceptional power to certain junglers that are good at controlling them (like Nunu) especially when late game rolls around and elders come into play. I'm not saying it wouldn't be a fun challenge to fight that. But for many it would be frustrating, and strategy-wise, it would take a lot away from the current JG in my opinion.