I fully agree that you can, and whilst the block outputs that it has set the player's spawn to whatever the relative command should be, (at least in the examples I have tested and in versions up to 14w30) when the player is killed thy are returned to worldspawn
Indeed whilst you can execute for spawnpoints, there is a particular issue that executed spawnpoint does not accept relative co-ordinates when run in an execute as detailed in MC-63964.
Nicholas Hills
I fully agree that you can, and whilst the block outputs that it has set the player's spawn to whatever the relative command should be, (at least in the examples I have tested and in versions up to 14w30) when the player is killed thy are returned to worldspawn