I get it was to set up the OCD and attention to detail.
The fly set up more than that. The Fly established two things:
1. Walt's obsession with perfection via his pride
2. And the smallest contaminant could ruin "perfection" ie Walt's plans.
This episode actually centers around Walt and Jesse, and their relationship, that's it.
And Walt's relationship with Jesse is the "fly", it is the contaminant. His loyalty to Jesse, is his weakness. In a sense, the idea was Walt should have put Jesse down early, but by letting him hang around, Jesse would ultimately ruin everything.
That's the irony and symbolism of the episode. Walt was REALLY lecturing himself about the dangers of Jesse TO JESSE and not realizing it.
This post was edited on 9/16 at 7:21 am