Peaceful mode stops
anything spawning that can be hostile (eg Endermen). Only passive mobs such as cows and Testificates will spawn. If you try to spawn something in (eg via a spawn egg), it'll instantly disappear.
Anyway, the commands aren't "cheats"...they're there for people who might want to turn the mobs off, eg if you're trying to just build something in Creative and don't want zombies and stuff randomly wandering about and getting in the way. Also, you might be trying to build something really cool in Survival mode and have it messed up by a Creeper, which of course can be really annoying. Though you can turn off the environment damage caused by a Creeper (they still hurt you, they just don't destroy blocks), as well as Enderman block-stealing abilities via another of the new commands.
Minecraft is supposed to be a "do what you want" game - if random-spawning mobs are appearing and messing things up for you, you can't really do what you want

. A lot of the changes made in the new patch (and in the one before that, and possibly the one before that as well) have clearly been made to accommodate Adventure mode, the third mode that was added in a patch or two ago. You can't break or place blocks. It's so that people can design their own adventure map for other people to play. Now you actually
can build a spooky-looking dark dungeon that's actually empty but the darkness makes it seem otherwise, because you can turn of mob spawning when the player enters, but then have a trigger than turns it back on again when they reach an area where you
do want mobs to spawn.
People are gonna be able to do so much regarding adventure-making since this last patch, without having to ask people to constantly change the difficulty level in the Options (something you can't do if playing online unless you restart the server) or worry that random mobs might spawn in a supposed "safehouse" or whatever

. It also means formerly-complicated stuff which some people got around by having hugely-complicated redstone mechanics can now be achieved with a lot less difficulty...and a lot less space

.
I suppose technically having mobs spawn anywhere it's dark can be
annoying, but the mobs are now just another thing you can include in the game, if you want to, in order to add to the challenge. Plus there's some items you can only get off certain mobs unless you spawn them in via Creative mode or the command to spawn in an item via its item number.