I've seen several companies think this is a good idea but in practice it's a really bad idea to do this because you get captured by the customer and effectively

I've seen several companies think this is a good idea but in practice it's a really bad idea to do this because you get captured by the customer and effectively become an arm of them without the benefits. And you end up dedicating so much time to them that other customers won't be happy and if that big customer ever cancels or leaves (which is often a board cost cutting decision rather than a user decision), then you end up absolutely screwed.