The hossy where I went used terry flats - I sucked at putting them on and so did DH, he turned up on the second day with a suitcase full of Nates MCNs (all his own thought lol). So he's been in them since day 2. Admittedly he's no lightweight, but he's just long. He still has skinny legs.
I had Darcy in them from 8 months and we never used a sposie after that. We travel heaps (we live 600km form our families) and I have never found it an issue.