Yes, of course I would and do—it happens all the time:
"This place has great coffee."
"Yes, it does. Smith's across town also has great coffee."
"Oh, they do. Good brownies, too."
The difference is that we are talking about places that provide great coffee.
DreamHost does not provide great email service. By its own admission, at that. Talking about email providers is not bad form. It's just current DH customers trying to find someone who is good at something that DH isn't. If I find someone else to handle my email, it's a win-win. I win because I get better email service than I'm getting now, and DH wins because there is less load on your server and fewer support requests to deal with.
Someone else has said that it would be great if DH partnered with an email company to provide email service that is as good as its hosting services. I agree, but can't even begin to imagine how such a partnership would be managed. In the meantime, we can have hosting at DH, and email somewhere else, and DH doesn't lose a dime.
Talking about other hosting providers could be seen as bad form, however. But as long as DH is still making the same amount of money, then no, talking about other email providers is not bad form.