I wonder where you heard it's a bad idea? Definitely not from me. I've been on both sides of the fence now, my prior rig an FL70 and now a class 8 Freightliner Columbia. I've gotta say I'll never go back to a class 7 after being spoiled with the class 8 power and luxury. My truck was not a stretch, however, it came from the factory with a long frame and just has 3 foot frame stretches at the rear to accomadate my 24' box.
Disadvantages? Really only one, it is front heavy. I usually start my heavier loads about 5 feet back from the nose of the box to keep from getting overweight on the front axle. But, sometimes class 7's have this problem too. I'm still running a single drive and no tag, and now that I know how to load the truck right I have not been overweight up front anymore. I also have dual 150 gallon front mounted tanks that don't help me- I just don't fill them all the way up.
Advantages? Too many to list, really just about everything. The class 8's are made for the highway, and make doing the job so much easier. Power, comfort, long life, etc-etc. I feel sorry for people who go out and spend $60K for a new FL70 Alumi-Junk expediter with throwaway components when I picked up my Columbia as a year-old repo with 54K miles on it for $56k. The Columbia will easily last 10 years or more should I decide to keep it that long, but I know I'll have at least 5 powerful, luxurious, quiet years with it that should be more or less worry free.
Before you consider stretching a tractor and finding a box, look on the web and in things like the Truck Paper for a used or repo class 8 unit. There have been a lot built at Freightliner of Orlando (Usually Centuries or FLD112's) that are not stretches that are out there if you look around.
If you can't find something, by all means stretch a tractor before wasting your money on a class 7. There are a ton of cheap tractors available, just pick a make and engine/tranny you like. Good used boxes can be had for under $10k, and a typical stretch job usually costs $5-7k. So if you buy a $40k late model used tractor, with the stretch and box you are still under the price of the underpowered noisey Alumi-Junk throwaway FL70, and you will have one hell of a NICE, REAL highway truck!