Artists that use the same song structure...

Someone once said that every artist or group is just constantly rewriting the same 7 songs over and over again. I agree with that to some extent, but I don't think it's bad, or even necessarily deliberate. It's just that artist's particular pallette of expression. Some artists are a little more limited with their bag of tricks, and some more deliberately experimental.

A few more come to mind - Norah Jones, Alicia Keyes, Enya

Remember when Mick Jagger sang "It's the Singer Not the Song"? When I was younger I strongly disagreed with that sentiment, putting the song way above the singer. Hence my love of The Beach Boys, precisely because they had the best songwriter in the world (Brian Wilson), but only okay singers (except for Mike Love, who was/is dreadfully bad).

As I've gotten older, my thoughts on the matter have changed. And though the song still comes first, singers are of much more interest to me than they used to be. Iris Dement, Emmylou Harris, and Allison Krauss from the gals, John Hiatt, Buddy Miller, and Jim Lauderdale from the guys, being amongst my current favorites.

Pop music songwriting took a giant nosedive in the 70's, when Rock Group's songs became not much more than a guitar riff. The "sound" of a Band became their "song", what they were known for. Bands like Alice in Chains have only one song, really, every single one even having the exact same two-part harmony (and it is not a very good one). Boooring!

All blue grass music has the same song structure and melody.

By definition, the vast majority of blues tunes performed by singers like SRV will have the same "form"; the classic "twelve bar blues". What distinguishes one blues song from another is typically the melody and the time feel; the "form" is usually (not always) exactly the same. The two songs you mention not only have the same form, as is usual for blues tunes, they are in the same key, the melody is very similar and the tempo is almost the same ("I'm Crying" is slightly slower); those are the things that make them sound so similar.

SRV is a blues/rock singer and his musical palette is fairly limited. I don't think that the fact that many of the tunes he performs have the same form is any sort of indictment of him as an artist. I do think that those two tunes are so similar that it seems almost ridiculous, but not terribly surprising. I do think it is an indictment that he is not able to do something with the interpretation of each to distinguish one from the other.

Some of the other artists mentioned do sound very similar from tune to tune, but those tunes don't necessarily use the same "form". "Form" is not the same as "formula". Actually, a better way of looking at this issue, and a great test of an artist's true talent, is wether an artist can sound fresh and interesting IN SPITE OF the fact that his/her tunes use the same form.