Never mind the fact that it makes for a good (and nerve-wracking, in my case) plot point, but why did the hobbit feel the need to take this drastic step?
Yes, I know that he has some sort of honour guilt for indirectly having caused Boromir's death by being protected by the same, but surely he would have helped in the same manner even if he had not officially been "sworn in" as a soldier?
He was a special guest to say the least, arriving with Gandalf and under his care, and coming at a very special time. He would obviously be of as much service as he could until the End, regardless of whether he was technically part of their army or not.
It just seemed out of character to me. And I felt quite uneasy about hearing how he got "duties" and almost downgraded himself from the status of a special friend of Gandalf's, to a "standard soldier" under direct authority of that rather mentally unsound Lord of Gondor.
I'm sure I'm probably overlooking major themes of honour and whatnot, but it just seemed odd to me for this little timid hobbit to so eagerly want to fight in this manner, rather than simply helping as himself, to the extent that he can help with something.
The same goes to a certain degree for Merry as well, but at least Merry has more of a friendship with king Théoden, who has not bound Merry to loyalty until death and warned of scary-sounding punishments for breaking his trust, as the Steward did.
Why could Pippin not just be a helpful guest rather than (seemingly) unnecessarily swearing himself to this rather unpleasant (if not actually evil) ruler?