It's clear what the advantage of two-piece gear is: easier resp. faster at sanitary stops.

I do LDs with two-piece gear: Castelli T1 Stealth top with Castelli Corsa Rossa trousers.

If we ignore the sanitary thing, why would I change to one-piece gear? The only difference would be the transition from top to trousers at the waist, isn't it? And you wear a numberbelt at most triathlons anyway which also disturbs the airstream, so I wonder if a change would make me even one second faster. Or is it that a one-piece suit will always have less wrinkles than a two-piece gear?

All I want to say is, that given the disadvantages a one-piece suit has, there must be a clear expectation that it would save me at least two minutes.