The reason to stay at the Swan/Dolphin is price. Also I can confirm their beds are WAY better than any bed I have ever slept on at Disney, and the rooms have MUCH thicker walls.
The thing to remember about Marriott pricing is unlike Disney, their rates are not fixed. Disney's room rates are fixed and will not go up or down depending on when you check them, there may be discounts but the rack room rate will be the same for a certain date whether you book it the day it's available or the day before it sells out. You can see the rate calendar for the year when you book a room, and Mousesavers has organized tables of all the room rates at every resort by room type and date for all of 2023.
Marriott prices are dynamic and are exactly like airfare, they change multiple times a day and will go up and down like a rollercoaster. If you check rates for your trip one day, they may be $100 more per night or $100 less per night when you check again the next day. The nice thing is when you book a refundable rate, you can modify your reservation on the website without rebooking, and the confirmation number stays the same (you link it to
MDE, and do not have to relink it if you modify your rates).
I'm one of those people that checks my room rates and airfare a few times a week, and it absolutely pays off for booking the S/D. I booked the Dolphin (with the AP rate) for $220/night INCLUDING taxes and resort fees over the first week of January, smack dab between two very busy weekends (NYE and Marathon Weekend). The regular public was a bit more but still well under $300/night.
Just two days ago, the public room rate for the Dolphin was $300/night including fees for Princess Half Marathon weekend coming up next weekend. I find the Swan is maybe $50/night more. In comparison, the cheapest standard view room (if you can somehow snag it) at BWV is more like $600-800/night for the same dates.
If you are paying $400/night for a standard room at the Swan/Dolphin, imo you are paying too much and should be checking regularly for better rates. I find the sweet spot for lowest rates has been 3-4 months out, and then a few weeks before traveling if rooms are still available (and availability does come and go even during popular times, because the resorts are big and many people cancel at last minute).