1. If the artist wants to... I think it's always acceptable to ignore grammar intentionally... that doesn't make it good. It depends... usually it's fairly obvious when someone just didn't pay any attention to the grammar and when someone meant to defy grammar laws. Lots of rap is great and there is not a damn bit of grammar in the best hip-hop.
2. If you can sing it and it sounds good and it gets the point across... who cares?
3. On that note... "reading more like a poem" is exactly that... reading. Songs generally are expected to have some flow about them but when we read certain lyrics, it will sound like a poem. It all depends on the song. Hm... I think of a song like Vacant by Dream Theater-- it's sung, it's a song, but the lyrics if read sound more like a poem whereas many other songs flow with lots of music.
It's not a bad thing to tell a lyricist that his lyric reads like a poem, nor is it bad to have odd meter. When i mention one or the other i usually do so because the lack of metrical consistency seems to hurt the overall flow when read. But always, i try to say that if it can be sung well... it's not a big deal.