I agree with you all that Angel was more useful (fighter-wise) as a vampire and such, but I always kind of pegged the episode a bit differently myself.
From http://vrya.net/bdb/index.php, "I Will Remember You"
MALE ORACLE: Why are you here, lower being?
ANGEL: The Mohra demon said the end of days had begun. That others were coming, soldiers of darkness. I need to know if he was telling the truth.
MALE ORACLE: As far as such things can be told.
ANGEL: What happens to the Slayer when these soldiers come?
FEMALE ORACLE: What happens to all mortal beings. Albeit sooner in her case.
ANGEL: She'll die? Then I'm here to beg for her life.
MALE ORACLE: It is not our place to grant life and death.
ANGEL: And ask you to take mine back. Look I can't protect her or anyone this way, not as a man.
FEMALE ORACLE: You're asking to be what you were, a demon with a soul, because of the Slayer?
MALE ORACLE: Oh, this is a matter of love. It does not concern us.
ANGEL: Yes, it does. The Mohra demon came to take a warrior from your cause - and it succeeded. I'm no good to you like this. I know you have it in your power to make this right. Please.
ANGEL: If I stay mortal one of us will wind up dead. Maybe both of us. You heard what Mohra said.
ANGEL: No, I won't just stand by and let you fight, maybe die, alone.
ANGEL: You know that's not it. How can we be together if the cost is your life, or the lives of others?
Angel's initial question was to know about the mohra demon's predictions, and the impact they would have on Buffy. He didn't even bring up the idea of changing back until they confirmed that she would indeed die in the end of days.
It seems as though he wanted to exchange his life for her own, like if he gave up his humanity Buffy wouldn't die. Which, since the oracles never actually stated otherwise, would make perfect sense. Given that he only started hearing of this AFTER he was turned human, it's only logical that if he could erase his humanity, he could also erase Buffy's impending death.
And it seems Angel just used him becoming a warrior for the PTB again as an excuse to get the oracles to grant his request, since they said they obviously wouldn't do it otherwise.
And afterwards, when he was explaining what he did to Buffy, although he did mention protecting others
("ANGEL: Buffy. You saw what happened last night. If anything I'm a liability to you. You take chances to protect me, and that's not just bad for you, it's bad for the people we were meant to help.") He also referred to her dying several times when they were talking, obviously showing his fear of just that, and how it had a heavy influence on his decision.
He would obviously do anything to keep Buffy safe, and if he knew (or thought he knew) that giving up his humanity meant that she wouldn't die, there would be no question in his mind as to what he should (and did) do. I mean, yes, he could have stayed human and trained to help Buffy just as Wesley and many of the Scooby gang had, but in his mind, that wouldn't be an option. Like I said before, all he knew was that the issue of Buffy's death came up when he was turned human, and that he had an opportunity to change that. No matter how much he wanted to, he wouldn't run the risk of Buffy's life to stay human; ESPECIALLY not when he already had a chance to do something that he was sure would keep her alive (again, this is keeping in mind that none of us know that Buffy ends up dying anyway)
Although his duty to saving others and redemption may have been on his mind, it seems to me like this was mostly driven by his desire to keep Buffy alive safe. He still has no idea of the Shanshu prophecy (or that he has a chance at "redemption"; or at least not a definite way of it- which the shanshu gave him.... you are redeemed- you get to be human) and I'm sure that if the mohra incident hadn't happened, he probably would have ended up training just as you suggested, to keep fighting the good fight.
It seems that Angel wasn't thinking cautiously much at all, simply doing what, in his mind, he had to do. He didn't question those things because he didn't think to. He had a problem (Buffy dying) and had a soloution (him taking back his humanity), for him that would have been the end of the story; no ands ifs or buts. No, it's not really all that logical, or rational, and probably doesn't make much sense, but it does fit into his character to a tee. It's just the kind of thing Angel would do, especially given the circumstances he was faced with.
As much as I would have loved to see a Buffy/Angel reuinion and happy ending of sorts, the end result of the episode did match the characters that Joss had written over the years. And by writing the storyline for that episode the way he did (with the second mohra attack and such), the writers really had no other choice.
