It's legal, but it's a reasonable request only in a situation in which the target is in doubt of his own judgement and has some reason to trust the caster. At all other times the target should have LARGE circumstantial saving throw bonus.
For example, its a reasonable request to a character that discovers that they have wandered into the middle of a minefield and has no idea how to get back out safely.
Another way to handle it is simply rule that such a statement, while legal (and probably unreasonable), effectively gives the target a saving throw each time the caster makes a new request, and give bonuses or penalties for reasonable or unreasonable requests as the new requests arise. Effectively, however worded, all such requests are identical to the statements, "I suggest you treat all my further suggestions as commands." or "I suggest that I am your master and you are my obedient slave." Generally speaking, that's not really any more reasonable than, "I suggest you commit suicide."