Ask HN: Thin-layer startup?

I know that sometimes building a product from the ground up is hard, but has anyone considered or do any of you use, another start up's infrastructure, and just add a layer above it? For example, I know some start ups are pursuing big data, if it was financially feasible, do you think it would be possible/legal/ok to use their services to store the data(provided through a proprietary transactional API) and offer services on top of that(analytic for instance)?