splits are almost too many games.
You may be right, I think but...
When I want to learn a new opening, I will focus on one line.
The problem is many thematic tournaments I see here are too general.
E.g., you have French Defense with one move position or Italian Game or Ruy Lopez with three moves position.
Really, too precocious, in my opinion.
In this case, you can't concentrate in one line if you have only SIX games in a quartet...
Your opponent may choose very, very different lines.
So, I think, a 'split' approach may be interesting for this kind of 'general' tournament but if you have, e.g. the Old Steinitz variant in the Ruy Lopez opening, the quartet approach seems really better, you're right.
I'm not fanaticv about split, e.g. if you have 8 (eight) players in a group, it may give a idea of this kind of general opening...