I was wondering if they really serve any practical advantage.
For one, what they mean depends on what the innate part of speech of the root is, which isn't marked except through the part of speech endings. For instance, attaching the verbal suffix -i to an adjective gives a different meaning than attaching it to a noun.
Further more, Lingua Franca Nova has a rather novel method of marking mart of speech: its own derivational morphology. Unlike Esperanto's affixes, LFN's affixes often transform a specific part of speech into another (or sometimes just leaves it the same). In this say, suffixes that derive nouns are automatically mark the noun for part of speech without the need of an additional affix that serves no other purpose.
But of course, I'm not exactly fluent in the language myself, so I'd like an opinion from people who do actually know and use this language. Are the part of speech endings really useful or are they just a convoluted way to derive new words?