Ew, what a disgusting title. But, sharing needles is what it amounts to.
If you're going to share email templates with somebody else, you're sharing in their reputation. Lots of good spam filters, like those at Cloudmark, Brightmail, Yahoo and elsewhere, they use what is commonly called “content fingerprinting.”
Let's say you decide to send an exciting third party offer to your email list. Maybe this is totally cool – maybe you've told everybody on the list you're going to do this, and all of your subscribers have agreed to it. And maybe you find some “partner” who offers you $18 for every single widget purchased by one of your subscribers. Your “partner” even has an HTML template you can use, no muss, no fuss.
But, if that partner has a bad reputation, you're going to end up with that bad reputation smell all over yourself. That's where sharing that content is like sharing a needle. If that partner works with a whole bunch of people sending email, if that template is out all over town, then there's a pretty good chance that somebody has sent emails using that template to poorly permissioned lists, causing spamtrap hits, spam complaints, and so forth. As a result, those smart content filters fingerprint that content, denoting that whenever they see that content, spam complaints are sure to follow. Denoting that whoever is sending that content is probably not all that good of a sender, and dispositioning that mail accordingly.
That's what makes it like sharing needles. Your reputation can now be impacted by two groups entirely out of your control -- the partner, and anybody else that partner is working with or recently worked with.
The net result here is that your deliverability suffers, even though it's not really your fault. Your list might be legitimate, but you used that bad content, the fingerprinted content, that the filters know to be used in bad situations. It can even result in your deliverability continuing to suffer AFTER you stop using that content – if the filter drags down the reputation of your sending IP address based on your use of that content, it's entirely possible that completely unrelated emails are going to suffer.
The best way to avoid this is don't dabble in partner offers, in my opinion. That might not be a real popular “sender” opinion, but I think ISPs would agree with me. What sets you apart from other senders is what you send and who you send it to. What you send being the key in this instance. If you send the same kind of garbage that other people are sending, don't be surprised if you get treated the same way as the other guys who are sending it.
If you do decide to promote somebody else's product or service in your newsletter or in an email to your subscriber base, make it different. Don't use somebody else's content. Write it up (and build the email) in your own words, in your own way. It will help to differentiate you from everybody else.