Hey Jonathans,
I had this thought when I first started out, seemed logical at the time. I can't speak for a HF saw but mine is also a horizontal/vertical and when I put the thinner blade on I noticed it wasn't tracking properly and by that I mean it was unusable. After some inspection I noticed the blade was riding funny in the bearings. Mine has eight bearings in the guide four double stacked and the blade was riding in between them. I tried tinkering with it a bit and finally gave up and put the old one on.
I am fortunate in that the folks that make my saw are a small shop here in the states owned by two self described "tool nuts". When I called one of the owners answered and he explained to me the blade had to be .5" wide for horizontal operation.
Yours may work out different but I thought I would share my experience.
-Clint