Clustering makes sense if all the transporters share critical resources - as they appear to do, because a single shot from Evil Kirk in "Enemy Within" disables the whole kit'n kaboodle.
OTOH, "Dagger of the Mind" requires one of the 6-pad platforms to be down on Deck 12 or thereabouts, a requirement FJ doesn't accommodate.
With four or perhaps five of the 6-pad units, this extra 22-pad unit barely doubles the evac capacity. Might still be worth the trouble. But we have conflicting evidence on how many people the total transporter system can handle. In "The Apple", Kirk wants to have nine people down on the planet, and they get there in two groups, 6+3. In "Day of the Dove", Kirk wants to have nine people up on the ship, 4 heroes + 5 Klingons, and they are beamed up in one group on a 6-pad station (but the Klingons are given the delayed materialization treatment).
Can the system handle more people than there are active pads? "Day of the Dove" says yes (even when "The Apple" leaves it unclear what is happening up in the ship, what hardware is being used and how, whether the nine really are in a hurry etc). So, can one transporter station operate the total resources of the ship? And if so, do the total resources equal the total number of pads (say, 6+6+6+6+6+22)? Or do they amount to less, or perhaps even more?
TNG systems can apparently do more than the pads indicate, judging by some cases where the explicitly only 2-pad runabouts handle three people. Again, TOS might be more limited. Even if ENT always wasn't...
Timo Saloniemi