If you're looking for rationale for not adding sway control, you've found it.
I've experienced sway once (not a TT, but towing a vehicle on a open trailer). Wasn't fun at all. Happened immediately once I started up to freeway speed. Got the vehicle under control and stopped, pulled the vehicle up closer to tongue (more tongue weight), and it towed great.
Having said that, I bought an integrated WD/Sway Control. Blue Ox Swaypro. Why not? If you say you need a new hitch, why not get as good of a setup as you can? Maybe/hopefully you will never need it. Hopefully I will never need it, or need my automotive insurance, or my life insurance... but I have it just in case.
As others have said, the "minor" movements one feels due to wind, trucks passing etc is not sway. I still feel some of that with my Blue Ox.
Mike