I'm in the "it depends" camp, too.
Not all dresses with spaghetti straps are slip dresses, IMO.
The true slip dress is pretty minimal and almost delicate-appearing, barely-there kind of clothing and kind of demands a more waif-like build, or just the right build at any rate, to not look like a VS ad at best or trying too hard at worst. This is of course my jaundiced eye at work, since I am not a fan wearing clothing that looks like underwear or even underwear visible. I think of it as date night/clubbing/special event (could be a long evening gown, for example). Certainly there have been gorgeous silk charmeuse gowns on the runways or red carpets worn by elegant (and usually young) ladies to great effect.
A dress that is more structured or fitted, but with spaghetti straps might or might not have those issues.
A lot of these type dresses are in fabrics that aren't especially cool (polyester) or practical.
Otherwise for heat I think it is easier to be successful in a tank-top style dress in a cool fabric and fluid fit.
But no harm in trying, as you may get the planets aligned in your dress.
