The definition of a preposition is that it connects words or phrases, so some say it shouldn't come at the end of a sentence because it isn't connecting. However, when prepositions are found at the end of the sentence, their function is normally different. If we say 'They had a fight, but they have since made up.', 'up' is not functioning as a preposition, but is mofifying the verb. Some call this a particle, but I prefer to think of it as an adverb modifying the verb (the OED follows this line).
Therefore, there is nothing wrong IMO with using such a word at the end of the sentence because they generally aren't functioning as prepositions when they take end position.
