Asking for sponsorship to speak: a lack of respect for the attendees
When people pay to attend a conference, they do it for a lot of reasons, but the biggest one is to learn from great speakers and experts in the industry. Yes, the networking opportunities are awesome. The party, if any, may be magistral. You may even have to travel to a new city or country, but in the end, it's all about the content.
When conference organizers offer speaking slot in exchange for sponsorship, it's a lack of respect for the attendees. You should aim for great content and great presenters to give the best show possible to your customers, the people who paid to have the privilege to assist your event.
Don't get me wrong, I've organized many events in my life from small to big ones of different types like conferences, and I'm totally aware you need money to run these. Depending on the experience you want to give around the presentations, it may mean big bucks! In my humble opinion, there are other ways to pay the bills, and continue to give a great experience. If they are the speakers you are looking for, accept their talks and discuss sponsorship after, so things don't get mixed up. In most cases, if a speaker is accepted, there is a big chance that the company sponsors the event: after all, they want to maximize their exposure. It's what we were doing when I was at Mozilla...
Similar to not paying non-professional speakers (not paid by their company to share their passion), I firmly believe it's a practice our industry should stop doing. After all, content is the key to the success of your event...