I don't have a problem with offering DMs. The recipient has the option to opt-out as a protest if they don't like it. Your stats will also show users if it is more bad than good presumably. You could also let users send an initial DM requesting whether or not the recipient would like to get further DMs. The main reason why you cannot afford to not offer DMs is another service will, and they then could scoop your customers.
That being said, we're definitely interested and proactively working on ways to provide value for users of our service and those that receive messages. I would say about 50% of the campaigns are sent via DM and the rest just to the public timeline. Our most valuable clients tend to use the one which makes the most sense to them.
We do have a few users that just spam their followers (per our definition of spam) and the updates listed in the other post will hopefully curb them off the site. Additional ideas would be great too.
I'd love to see Twitter provide more granular control over direct messaging and allowing you to follow a user but not accept DMs from them.
Thanks for the constructive criticism!