I think some grammar errors can be intentional, stylistically important, and colloquially important, among other things. It can also be a royal pain the butt to trudge through, it just depends. A level of understanding is incredibly important, and unintentional grammar errors can be remedied easily. I think what's more important is knowing whether grammar is a weakness for you, and tending to it properly if it is (via editting and/or education).

I think there are "poor grammar" (things) for lack of a better word, that help portray what the writer feels is important...
I am guilty of poor grammar. I have been since I started communicating via text. Often times, it's due to time constraints, wherein it's faster to type (or tap) "lmao" rather than Laughing my ass off.
From there, it's all downhill...  
I will use the word "ain't" if it flows better than an alternative.
I also use poor grammar because it seems 90% of the people I know use poor grammar, so I figure... wth.
As much as I use poor grammar, and enjoy it for a multitude of reasons, I do enjoy good grammar, and I love great grammar.

In writing, and other arts, I push myself sometimes to continue experiencing something, even though it may not be my bag.
As long as the grammar isn't so terrible that I can't make it out, I think feelings can still be conveyed given the chance.
For some people, writing in bad English is their only outlet, I try not to shun anyone. ;)

snugglebuck said:"Number one rule of poetry; there are no rules in poetry?"

Dig?

I take it you never heard of iambic pentameter

Personally, I always try to use correct grammar. It's always nice to read a poem that sounds and feels polished. I think an exception would be if it conveys how you are feeling. If screwed up grammar displays how you feel, than do it.

If the errors are intentional, as in some conversational or dialect pieces they can and often do enhance a piece.

When  it's fairly obvious that they are not intentional they do detract from the piece for me.  Errors here and there are normal however, and I will point them out to those I believe would wish to correct them.  

If sloppy grammar is a pattern across a poets work and they never bother to correct and/or show umbrage at corrections I will stop reading their work because to me it shows that they care only for the attention they hope the piece will bring and not much else, which makes me unlikely to connect with anything they write. Caveat, I do take into consideration and give much leeway to those whom I am aware that English is second language.

It is my own personal theory that those who say grammar is NOT important are the same who never took interest in learning proper grammar anyway, which would explain why grammar, to them, would mean nothing.  However, to those of us who DID take an interest in learning proper grammar, (even if we are still in the process of learning or perfecting it), can easily understand why grammar IS, indeed, important.

I agree...If the poem is purposely composed with bad or improper grammar/spelling to convey some particular point, that would be the poem's art, otherwise improper grammar just because "grammar is not important so I don't have to look over my work" does steal from the poem and, to certain extent, leaves the reader confused where only the poet, himself/herself, knows what the message or point of the poem was because "Grammatically-Incorrect" poems have abstract fragments of possibly many thoughts blotted on paper without structure or form...just blot of a brainstorm!  It only majkes sense to them alone.

(This is generally speaking, not offending any particular individual).

When I write my poems, (though I'm very simplistic-not a Shakespearean), I want my readers to understand it fully and let my words and thoughts flow like a stream as readers read through it.  It's NOT my reader's job to add adjectives and words to configure or guess what I'm "trying" to convey.  It's poetry, not a game of MadLibs.

I do care about Grammar...but I am not OCD Grammatical sufferer. However, a poet who has a trail of grammatical errors should be VERY concerned about Grammar if the poet should, one day, decide they want to publish or self-publish their poems into a book. For one, I would HATE for the poet to be humiliated or ridiculed when his editor has scribbled all over his manuscript endless grammatical corrections. To me, if I consider myself a poet, I consider myself a writer...and if I am a writer, I should exemplify that I have writing SKILLS, not EXTREME lack of it.

IF my SELF-published book makes good in sales, I would HATE for anyone to pick up on my errors and think that I am a SKILLED writer, so my grammatical errors are actually accurate.  I learned how to express myself verbally and in writing by the lots and lots  of reading I did since my younger days!  Books were grammatically flawless so I captured many uses of words and how to properly use words that sound the same but are spelled differently, like to, two, and too).

Will I want someone picking up on my errors adapting them as truth?  No way!  I want to be an author or a poet readers can look up to....be a role model and have them learn proper word structure and usage from my skills, not my doodles.