I am curious to know what the general consensus on the limitations of this spell is. ie:
What does "obviously harmful" mean?
What does "sound reasonable" mean?
ex. Your party of three bursts into the lair of a known diabolist to arrest him. Seeing no obvious threat, you start searching the place. While searching a pack (5-7) of lesser devils start bursting out of boxes and surround the alchemist, a fight ensues. During the fight a sneaky Imp comes down from the rafters and casts suggestion on the ranger, telling her "Your outnumbered, go find help". Ranger runs away to find help leaving her two comrades alone.
So the question is, is running away "reasonable" or "obviously harmful"?
As you can probably tell this is a personal experience of mine and I'm a little jaded on the subject which is why I'm posting here to hear what y'all have to say.
As a DM I'd call that kind of on the bubble, and I don't know that I'd do that to my players. (As the person who's the final word on what counts as what when it comes to mind-affecting spells, I tend to keep NPC uses of the spells to things that are clearly acceptable.) The thing is, in any even remotely dangerous situation, there are Suggestions that can be made that are vaguely reasonable-sounding, but that are also, with meta-knowledge, highly likely to result in a PC getting creamed. Particularly in a three-person party like you describe in an encounter that's already somewhat challenging, removing a combatant from the fight makes the encounter dramatically more difficult.
Honestly, I think it depends on how powerful of a combat spell you want Suggestion to be. It's not like taking someone out of a fight for an extended period of time is totally unprecedented for a level 3 spell, but it makes Suggestion pretty powerful, especially for enemies. (PCs are rarely immune to mind-affecting or language-dependant abilities.)
That said, if you look at other level 3 spells that remove a character from combat, they tend to last for rounds/level or are reversable by mundane means. While spells are allowed to be better or worse than other spells, I'd generally only allow Suggestion to be usable to remove a creature from combat entirely in circumstances where that's a -very- reasonable course of action.
Note that that doesn't make Suggestion a worthless spell for combat; I'd still allow it to be used to manipulate enemy tactics, for example. ("Why aren't you attacking that guy instead?") This is on top of its extremely versitile out-of-combat use, which comparable spells (such as Deep Slumber) don't quite have to the same degree.
I realize that this is justification based more on metagame reasoning about spell power than on pedantic "the dictionary says X means Y" reasoning, but there really are a wide range of things that the spell description might legitimately mean, so I'd choose to go with a line that places the spell in a reasonable context within the game.
Reasonable, to some extent, depends on the target's natural habits and thought processes. Suggestion isn't dominate. I would say, for instance, that suggesting a cowardly kobold run away would be reasonable, while suggesting that an Orc barbarian who loves combat do the same would be unreasonable. Getting drow or devils to backstab each other in combat would be fairly reasonable, while angels wouldn't do that. The GM needs to have a clear idea of the personalities and characteristics of the monsters for this to work, and the more the player knows, or the more plausible he can make his suggestion, the better. For instance, "run away" might not work with a disciplined hobgoblin, but "these guys are too powerful, run get help" might very well work to produce the same effect. It's all in how well you present it.
Obviously harmful would include things like "jump off the cliff" or "walk into the fire"' but perhaps not "hold off the ghouls for just one more round while I go get help". If it causes immediate hit point damage, it's obviously harmful.
Generally I handle stuff like this by giving monsters a bonus on their saving throw that scales with the implausibility of the suggested action. Note that charming someone first and then suggesting something has nice bonuses as people usually trust their friends. Eevn with that combo and good Bluf/Diplomacy skills, though, plausibility and Sense Motive are your friends.