I consider stances critical for development of strength, flexibility and proper footwork for delivery of attacks and effective defense. Once these abilities are well developed, their practice can continue to allow further improvement.
Like any technique, they provide the tools that you can pull from to respond, not the exact response, which should be geared fit the situation. We consider that stances are present within any type of movement, even running (you may not see the tiger stance - cat stance transition, but it is there). They can be used as footwork either high or low to deal with nearly any given situation. However, I would hope to NEVER see a student using specific stances during a street fight, mobility is too important. We maintain a clear separation between technical and practical training. Though the abilities developed through long-term technical stance practice will allow them to better deal with any given practical situation (many of which you might not normally expect to end up in).
I don't consider stances or basic techniques a handle to hold onto or a prop. Many students don't know the options and don't have the tools. For these students it is important that they train stances until proper footwork becomes natural and spontaneous. For other students who naturally have exceptional physical abilities and great footwork, possibly one could argue that they would not need to train stances. However, I have never had one of those students. I assume that they are very rare, but maybe they just don't come to me for instruction, maybe they don't need martial arts training?
Just my opinion, I am curious what others have to say.