I think the Say You Will cover is the the most beautiful they've ever done. It is very symbolic.
Lindsey and Stevie are on the cover, facing away from each other, both of their left hands visible. Lindsey's wedding ring is visible, as well as the diamond that Stevie's parents gave her. The rings signify unity. Stevie has her eyes closed, which to me, signifies she is still "dreaming," while Lindsey is wide awake, "reality." Lindsey's songs on the album have the "realistic" tone while Stevie's still have a "dream-like," romantic quality to them.
Mick and John are on the back holding hands. To me this signifies strength and unity. Mick and John are the only two remaining original members of the band. I see nothing weird about them holding hands.
I love the SYW cover. I think it's is open to a lot of interpretation, way more than any other cover. Just my opinion of course!