A few times I buy Fleet at different times than strictly as a duchy replacement:
1. Extend my 3-pile options. For instance, I Stonemasoned two provinces into Border villages/draw, bought Fleet & cleared out 3 Smasons for the third pile, and then turned the Bv into 4 duchies during the Fleet turn and bought a Province. Fleet can let you bank the value of green cards as something game ending (e.g. remodel a Colony into a Plat) and use it again in the Fleet turn it back into VP after triggering 3 piles (Remodel Plat back to colony).
2. Build out for a long shot green destruction megaturn. The most obvious here is mass play of Swindler. If I am down 4 Prov, but have Kc/Swindler, I can get a lot of mileage out of emptying the prov pile and then trashing provs into nothing. Similar options can occur with Knights or Giants and if it is a last ditch option, it often is worth skipping a province to be sure to have another turn to trash green. Playing 9 Swindlers with an empty province pile is just brutal and easily worth multiple colonies to get a clean turn at green killing.
3. Be sure to spike something in the deck for a bunch of points without having to carry it in the deck. The obvious case here is Fairgrounds, being able to leave Curse/Estate in the supply while gunning for 8 VP Fairgrounds is worth a lot more than a duchy and hence I often buy it at any point I am unable to buy a Fairground. Likewise, mass gaining Caches for a few Triumph finds Fleet to be worth more than a province often. Bishop likewise can often spike a lot of VP on the Fleet turn and make securing Fleet competitive with buying Provs or Colonies. Likewise, being able to overdraw and use a bunch of newly gained Transmutes can power up Vineyards while destroying the deck. If you can score huge points by sacrificing your deck, Fleet can easily be worth more than a colony.
On the flip side, I have a had a few games where Duchy is better than Fleet. Shepherd/Xroads was pretty obvious, but I have gone for duchy first with Crop Rotation and a few other green enabled options.
I figure you will go right about 85% of the time in two player with the "buy Fleet before your first duchy" option, but there is a nice minority of games where you either want Fleet a lot more than that or where by the time you see Fleet there are no duchies left (Tournament, for instance, makes Fleet often turn into a booby prize that is lucky to generate 2 VP).