As writers we should endeavor to pay attention to people, and not just their most superficial characteristics.
I co-wrote one story on E where my writing partner was very into phonetically written accents. I was happy to oblige her and write such an accent, but even as I was doing it I was thinking, gosh this is butchered, I'm making such a mess, this will be the time she asks me to just stop already. I would ordinarily never write that way.
Where I normally see phonetically written accents are among beginning writers who are in their special snowflake stage of character development, where there is a desire to make a character quirky and different, instead of making her relatable and genuine. It is easier to write a caricature of a person and harder to communicate who this person truly is. If you are serious about writing, the hard route is the way to go.
To give a character a voice I think it's important to pay attention to their vocabulary, their turns of phrase, their sentence structure and how they think about things. Words are our way of expressing the way thoughts occur to us, and we are so, so different in that regard. Even though a bunch of other posters have largely agreed with my thoughts here, my post is noticeably different from any of them. It's solely my voice that makes it so.
And see, I did not even have to adopt an accent. Would my post have been more interesting if I did? Or would it merely have been harder to read?