Well, correct me if I'm wrong. But I can't see how is it possible to be relaxed and stretch my hand doing a spider excercise at the same time. It takes a lot of effort for me even around 8-12 frets now. Shifting it all the way is not a solution if I want to play difficult stuff in future, right?
Well you said it yourself.

I definitely advice against stretching and spidering. Key to playing with souplesse and accuracy is to avoid strain, and stretching/spidering does just the opposite.
And the chance you will actually have to play something resembling spider in a real musical context is close to null.
On the other hand learning (new) music, transcribing, practising basslines or runs that are taken from songs you like, will enhance your playing and understanding much more.
And, will most likely not drain your endurance so much. Like others said, if you work on something interesting you're probably able to spend more time on it (if you wish to).
That's how I look at it anyway.
Oh, and yes, shifting is the solution when playing difficult stuff, in most cases. If you have to stretch, it will only be for a split second. Again, moving your hand around the neck causes no strain, stretching does.