In my opinion asking a creative vendor to do free spec work to get the job is unethical and also discounts the most valuable part of their service. The discovery, strategy and brainstorming needed to produce branding assets that will meet and hopefully exceed your expectation takes time and expertise and is what ultimately leads to a great result. In addition, if a vendor is willing to just whip out some "ideas" for you, they probably do not have an established methodology for delivering consistently amazing results. On the other hand, if a vendor can deliver a documented branding strategy methodology during the sales process and insists on you paying to have that methodology applied to your project, they are much more likely to deliver excellent results, on time and on budget.