Well of course your mechanic would say so; that's 2 full changes for him to charge for.
Check the tire manufacturer's website for reliable information. MODERN radial snow tires are tested for durability in standard and snow conditions, so they can stand the heat fine from a failure standpoint. Most of the wives tales are from old bias ply tires of the 60's. The soft compound does wear more quickly as noted.
Absolutely nothing wrong with leaving them on, and will provide a margin of safety across the upper mid states if there is snow. Coming from Ottawa, we hit a lot more mountainous terrain at Northern latitudes (NY and PA, a bit of VA) and have lake effect snow across upper NYS. From Toronto on the I-75 route you don't have as much mountain to deal with in the North, so less likelihood of altitude snow, but Buffalo area can see a fair bit of lake effect.
However, be aware that snow tires do not behave as well on hot pavement and generally have poor wet handling characteristics. A rainy FL month could be an issue. I have 3 trips south on my Bridgestone Blizzaks (2 to FL, 1 to SC with another SC trip coming in 4 weeks). Last trip the temps in FL were in the high 90's all week and no problems, no appreciable wear, but they were very "sticky" on the pavement. I have much more mileage on my winters (>60,000km) than I do on my all-season summers (OEM Goodyear garbage), and the snow tires have at least 2 more winters left on them, the all-seasons are being changed in the spring despite their low mileage (<40,000km).
ETA: Here is the link to a more recent version of the Bridgestone page on winter tires in summer, as they say, faster wear (and higher cost) and lesser dry performance, but not dangerous.
http://www.bridgestonetire.com/tread-and-trend/drivers-ed/driving-winter-tires-year-round
And a statement from the Globe and Mail as to safety:
"There was a controversy in Quebec – where, by law, winter tires are required on all cars between Dec. 15 and March 15 – over whether winter tires were less safe than all seasons on summer roads. A study by the Quebec Provincial Police showed no significant difference, says the Canada Safety Council “The difference was marginal,” says Raynald Marchand, general manager of the Canada Safety Council."