The most compelling argument thus far for no Splitter in the starting lineup/being paired with Duncan is the bench problem. So I definitely feel like that's one major reason pop isn't rolling with Duncan and Tiago. However, I personally favor the Duncan and Splitter starting line up with those two getting most of the big men minutes.
I feel like the pros would outweigh the cons. Our best defensive front court starts and sets defensive tone for the game. Our offense would be more than fine with Ginobili back, Duncan playing the high post (which hes been doing for most of the season), and we could even move RJ to the bench and have Leonard start as well. And you could still stick Tiago with that energy lineup of Neal, Green, Anderson and Bonner. Unless Tiago has cardio problems (which I doubt and could actually be fixed if he played more), I don't see why he shouldn't be getting 25-30 minutes a game for the rest of the season. He's proven to be that kind of player. Even on off-nights where his shooting was off, he still found other ways to contribute (offensive rebounds, solid defense, hustle, etc.)
Our 2003 championship team didn't really have a deep front line either. Outside of the starting duo of Duncan and Robinson, it was Rose who would take the majority of the remaining minutes. Next in line were Danny Ferry and Kevin Willis. Both Ferry and Willis played limited minutes that season. So it was basically a 3 man rotation of Duncan, Robinson and Rose.