I'm not questioning you personally on this, but using your statement as a starting point for this idea.
One possible issue/problem, as I see it, with the "spiritual but not religious" is that organized religion offers ritual, and humans seem to be hard-wired to ritual. Now it is true that someone could create their own spiritual rituals and perhaps that would satisfy the seemingly innate need for ritual. I look at ritual as a way of approaching the divine that is beyond words and rests only in symbols. Language is a wonderful paradox in that it is both broadening and limiting. It broadens our world because we can name and identify and convey. But it limits because that which we cannot name and identify and convey is beyond our grasp, our understanding, our reality (cue @mfbukowski and some Wittgenstein quote he'll find for us).
God is beyond language. An infinite Being, the source of be-ing, the ground of reality, cannot be constrained within the circle of language. Every word creates a duality: this/not-this. God is underneath that duality or in that duality or however limited way we can describe it, and therefore unreachable by language.
Cue, therefore symbolism, which speaks to us beyond language. Cue the poets! Remember that scene from the film Contact where Jodi Foster's character, the logic-loving scientist, sees the beauty of the universe and says, "no words to describe it. They should have sent a poet!"
Poets can bend language, play with language, take language out of itself and try to convey what cannot be conveyed. There's a reason why St. John of the Cross, the Mystical Doctor of the Church, wrote poems to describe his mystical experiences of unity with God.
And ritual, especially ritual steeped in the symbols of the culture that gave us our worldview, can use those symbols to go right past language and zap us in our souls. Ritual is symbolism in action. It takes us past language, past conception, and allows the opportunity for that which is beyond words to reach down to us and us to reach up to that. God's finger reaching towards Adam's finger (an image that is itself a symbol).
It seems to me that spiritual but not religious is missing this important aspect of spirituality: symbolism and ritual.
(on the other hand, a dose of LSD could take you right there )