If you had a dedicated IP, which serves the purpose (to a degree) of obscuring the Marketo origins of your emails, then it could be questionable to opt back in to being Marketo-connected.
Otherwise, you'd be getting blocked by IP and/or envelope sender anyway, not by a 3rd-party DKIM signature.
Signing an email shows the recipient that there's someone taking end-to-end responsibility for the email content (as opposed to merely permitting the email server to use a sender domain, which is the best SPF can do). If the signer is the same as the From: address domain, that's considered a first-party or "author" signature -- the person who authored the content also signed it. That's the best use of DKIM.
If Marketo is throwing in a 3rd-party signature, that's not going to hurt anything unless the recipient blacklists or negatively weights by DKIM domain. That's pretty rare, but it can happen. DKIM is more often used for positive weighting. It's also a great tool for selective whitelisting because a recipient can say they allow emails that are *signed* and thus attributable to a given domain as opposed to emails that *claim* (via the forgeable From: or MAIL FROM:) to be from that domain.
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