I think in most cases I would consider it an abuse of my trust if a company I followed would send me marketing related DMs. Following on Twitter is NOT an invitation to send me DMs. It means - "ok, I am reasonably interested to let your blurts flickr across my screen, and perhaps by chance I'll catch something interesting now or then". If I want more, I can subscribe to a newlsetter or something.

DMs are extremely intrusive, as they also trigger a notification email. They should be reserved for stuff that needs to be private, or is urgent. Both doesn't apply to marketing blurps.

Why the hate - because DMs multiply the cost of following somebody by a lot. Whereas initially it is just one click "oh heck, why not", it ends up being a hassle of a LOT more clicks and wasted brain cycles.

Edit: why I said "don't advise your clients to do that" - my most likely reaction would be to unfollow that user, and I think many feel the same. Of course perhaps marketing will find that it is still worth it (just as newsletters work, even though everybody hates them).