I agree with the observations that extra nacelles also increase the likelihood that something will go wrong, whether mechanical or hostile.
Funny, I do not. It's good to have spares in case you need them. Maybe that's the enigmatic purpose of the third nacelle.
Esentially I don't think the nacelles are related to speed but rather to payload and therefore power.
I assume that the prominent purpose of the Miranda Class in the 23rd Century had been to transport up to 4 cargo containers (basically Starfleet's "workhorse"). Freed from transport duty you'd have a formidable frigate kind of vessel (no engineering hull to drag along like the Enterprise
) capable of continuing service into the 24th Century.
Constellation Class (e.g. Stargazer, Hathaway
) may have been designed to take over transport duties from Miranda Class but already in the late 23rd Century required more power to do an adequate job (hence 4 nacelles).