If an employer offers something intangible, unquantifiable, or that he may have no control over, he’s not really offering anything. In essence, he’s creating a one-sided contract where you do all the work and receive none of the benefit. That’s called an “illusory promise” and it makes the contract unenforceable. Exposure in particular is illusory because there's no guarantee that you’ll actually get it (i.e. that anyone will actually see your work) or that the employer must do anything to ensure your work is seen by others. Ultimately, the problem with an offer of exposure is that you end up pinning your hopes on a promise the employer may not be able or willing to fulfill.