Let me give my general opinion about walk-offs: Banned. Banned banned banned banned banned. Cheap kills are exactly what you say they are, cheap kills. There is a high risk-high reward, but there are also characters like sheik who could carry a good amount of the cast right offstage with a string of Fairs. I'm going to use Sheik as an example because she is in the easiest to get my point across with:
Shiek's setups into 'Fair Wall':
1. Any Fair confirm in neutral
2. A grab (percent dependant, but the lower the better)
3. F-Tilt
4. D-TIlt
5. Jab --> Grab (not guranteed but timing for escape is tight)
6. Probably a lot of other things, I don't play sheik that much
So sheik can use any of these in neutral to set up for a Fair wall leading directly offstage, and get kills anywhere froom 0% to I'd say about 60%. And that's using a string that has very strict timing. Any throw with decent base knock back (a few that come to mind are Sonic and Pikachu's B-throws, as well as a pivot F-Throw from Ness) could get
easy easy kills at absurd percents.
As for simply banning walk off stages, there are several in the game (Colisseum, Mario Galaxy, Wii Fit Studio, etc.) so banning them all would either be impossible or grant your opponent any other counter pick like Halberd or Delfino for other counter picks. While it would buff certain characters, it would make others untouchable. In a two-stock meta walk offs would be simply too powerful. However, if the solution to too quick matches would be to increase stock/time count, a good camper (like Villager or Sonic) could simply pick any other stage and have timeouts galore.
To sum up my opinion on walk offs, I don't think that they could ever hold a place in any tournament if other stages were also legal. Games would be dominated by certain characters and low tiers would be left in the dust even more then they already may be. If I come off as a little strong with what I've written, good, because walk offs have no place in a tournament. But that's just my opinion