DKIM provides sender attribution (for both spam and not necessarily spam)
The presence of a valid DKIM signature on incoming email doesn't mean anything much about whether or not it's spam, or even if it comes from dedicated spam senders. Spammers can and do add proper DKIM signatures to their messages, and many legitimate senders don't use DKIM or don't have valid DKIM signatures, as our recent DKIM stats demonstrate. For that matter, some spam comes from legitimate places which DKIM sign all of their outgoing email (such as GMail). However, it has recently struck me that what a valid DKIM signature does provide is attribution.
If we receive a piece of email with a valid DKIM signature, the DKIM signature means that we can confidently attribute it to the signing domain. Either it was really sent by that domain or that domain has lost control over either or both of their DNS and their DKIM signing keys, and one of these is far more likely than the other. With a valid DKIM signature, all arguments related to the real sender and backscatter and so on are swept away; it was real email from the sending domain, period. In fact the sending domain went out of its way to make their email attributable to them.
This doesn't mean that the sending domain will accept replies and bounces to that email; far from it. But it does mean that the sending domain can't argue that they didn't send out the email and so are not socially obliged to accept replies. They really sent that email, in a way that provides undeniable attribution. Any refusal to accept replies is just a middle finger extended to other mail systems on the Internet (a fairly common middle finger, of course, because a lot of the modern Internet is defined by not caring about other people).
PS: It strikes me that this attribution may be one reason that large email providers such as GMail increasingly want DKIM signatures these days, because once you have definite attribution for incoming email you can do a number of things based on that with much higher certainty. And people sure can't argue with you about email 'not really coming from them'; they signed it.
(This realization was sparked by a discussion with Aneurin Price in comments in this recent entry. In a sense it's an obvious one, since DKIM's entire purpose is to validate email as coming from a specific source and the flipside of such validation is necessarily attribution.)