A proper sentence should have both a subject and a verb, with the necessary clauses to make it complete and able to stand alone. There must be at least one independent clause. There must be a subject-verb relationship -- something or someone, doing something (or someone), all in phrases and clauses joined by adverbs and such.
In poetry, writers tend to feel that it's acceptable to use fragments. I could find dozens of poems on MTC right now that I could use as examples. Unfortunately, as much as poetry is an art and a conveyance of some feeling or impression, it should still follow grammatical rules.
For those of you who are thinking how poetry doesn't follow rules, how it's art expressed from the depths of your soul and rules simply do not apply to emotion or art, consider the effect that fragments have on the overall piece.
Let's start with an example:
Now this is nothing more than 4 incomplete sentences offered up in list form. There are three lines that have -ing form verbs, with no auxilliary form and therefore are not true verbs as used here. They SEEM to be a subject (wind/hair/tears) with a verb (blowing/streaming/flowing) but cannot function as such.
This is not poetry.
This 'piece' never gives the reader anything to hold onto, or to follow. It's 3 partial images, and a label.
There is nothing concrete, no place for the reader to follow the writer in order to see his 'vision' of lost love.
Now let's finish the sentences, make it more complete.
face to the wind,
with hair flowing behind her,
as her dreams had.
Her tears flowed, flooded
the dam of the river
of lost love.
OK so it's no masterpiece, but you can see how the reader has more footing in his trip through this snapshot.
The grammatical rules that poets tend to forego for the sake of 'art' only lead to better art when understood and properly applied. So if you feel the need to rebel against them for the sake of rebellion, consider whether it's worth allowing your work to suffer for it.