Most all my songs are my own versions, and I rarely, if ever, use an intro, or any ending except the short one. I just play the intro usually using variation 1.
However there are certain songs that have signature intros and riffs and if you can't play them, using the intro would be necessary. One song that comes to mind is Sweet Home Alabama. Hit that intro/lick and the place explodes! Maybe you have to be down South though! And, you can use a generic style for that song, but it's hard to keep playing that riff over and over while singing, so I use a style that was made from a midi file when I do it.
Without a doubt there are other songs that are like this, where you really need to either play or the signature riff or have it in the style/midi file to sell it. . . Pretty Woman, Grapevine, Amarillo By Morning, Suspicious Minds, Wonderful Tonight, New York, etc.
DonM