Bringing in an alt into a SM/dungeon with your main to "only revive" yourself is still considered an unfair advantage vs a player who does not bring that alt to revive themselves.
The main issues that are against ToS involve an Unfair combat advantage/multiple rewards with very little extra effort.
What about with lots of extra effort? Can I have two mains, an elf and a giant, and have them go on adventures together?
What if I have my main run an alt through Glowing Garg to get that alt trans?
What about ranking Rain Casting on an alt and using it boost my main's synth success rate without spending all that AP?
What about using advanced feathers with an alt instead of asking in global?
Oh, and I mentioned being locked out of content earlier... what about doing Taillteann Defense for G9? To do that alone you would need to have all three characters participate in combat, not just drop your main into the mission.