Why would he risk that and file this with the court???
To establish that he is opposing counsel, which is a point of consideration in the issue of whether or not he may be forced to testify in the criminal case. O'Mara has appealed Nelson's order precluding him (O'Mara) from deposing Crump, and Crump thought it would be beneficial to have something on the record that tends to establish he is opposing a "not guilty" outcome in the criminal case.
From a legal point of view, I think it was a stupid move on Crump's part. Any court would accept that he is opposing counsel of a sort, just based on his statement of intention to sue Zimmerman.
From a PR point of view, well, read what people are saying about the case and about O'Mara, based on the filing. If nothing else, Crump has succeeded in creating a dense smoke screen.