How are Grignard reagents stable in an ethereal solution?
We tend to assume that acid-base reactions are instantaneous. But they need to go through an activated complex of higher energy, like every other reaction. 

These reactions are usually done at - 78°C. As long as the energy provided to the system is below that which is required to reach the activated complex there should be no reaction. 

I know for sure that tetrahydrofuran is fully deprononated by nBuLi at room temperature. The color turns deep yellow, if I recall correctly.