I've been writing songs for a living since 1985, and have cowritten hundreds of songs with other writers during that time. All I'm telling you is what I've always done, what all of my peers have always done, and the general way the business has always been done, and provided the Reggie Young example.
If you feel morally compelled to give someone writer credit, knock yourself out. It's really that simple, it's your song-your choice. Over the years I've had artists record songs of mine and make changes to them (Trace Adkins & Ricky Van Shelton made substantial changes that I disagreed with) without asking for or receiving songwriter credit. I've also had artists/producers try to 'strong arm' me into giving them writing credit on songs that they've made no change to whatsoever, with the stated threat that if I don't, they won't record the song(s) - in which case I've always said no, usually in a pretty colorful manner. I've also had a substantial number of artists record songs of mine and copy the demo I produced, on more than one occasion note-for-note. The musicians who played the signature licks/solos didn't get writer credit, and I didn't get production credit on those projects either.
For me personally, if I sit down in a room with someone to write, and a song gets written during that time, they are a co-writer on it - doesn't matter if they don't have a word or a note in the song, my feeling has always been that I wouldn't have written what I wrote if that person hadn't been there...maybe they said or did something that spurred a thought. Not everyone does it that way, that's just me.
Bottom line is, if you genuinely feel the person in question made a substantial contribution to the WRITING of the song, by all means give them a piece of the writer credit. But understand there is a significant difference between WRITING and PRODUCTION. Coming up with a great solo, or intro, or turnaround, or ending; a grooving bass line or drum pattern; adding pushes, stops, or any dynamic movements - things like that are production, not songwriting - and it has been my experience, and is my opinion, that those do not warrant songwriting credit, unless the musician(s) in question are willing to waive their payment for playing on the project as side men.
Hope all that stuff clarifies what I was saying. Cheers.

http://www.rogerbrownsongs.com
http://www.nashvillesongwriters.com/roger-brown