I highly doubt it is about grind, it is about time spent playing.
What does sound better? Solofarming Smites for about an hour or fighting your way through the underworld for about 5 hours. Let's assume both net you the same amount of ecto (very optimistic).
Yeah, sure thing the whole 5 hour things is inferior and stuff. Sure

The suggestion made is achieving three things. One of which is making sure the Underworld Trip pays off to just everyone on the team. Not the typical crap of one person bringing 20 ectos home while the rest just got nothing.
The second thing it does is nullify the need to solo farm the Underworld. When you can get just the same amount forming a team or even more... why should you UW Farm then?
And the third is, it might balance of some quest rewards. Seriously, some of which are just insanely long and barely rewarding. You can tell which one i'm speaking about can't you? Does Grenths Monuments ring something? This quest will take you about 3-4 hours to complete - if you're in a haste what most UW Teams aren't yet all it does is net you some minor experience points. Why minor? Because you could simply leave and rejoin another tombs team after the first three quests without doing that long term quest. The rewards would be a multiple of it...
Seriously, tell me why farming and grinding is superior to forming a party of 8 and fighting for several hours? This is not reducing the amount of time you need but it is reducing the need for grinding and the frustration coming from unbalanced loot drops.