Item 601(b)(5)(i) of Regulation S-K requires that all filings under the Securities Act of 1933 include as an exhibit an opinion of counsel regarding the legality of the securities being registered, indicating whether they will, when sold, be legally issued, fully paid and non-assessable, and, if debt securities, whether they will be binding obligations of the registrant.  Because so many registrants are incorporated in the State of Delaware, corporate lawyers in California and other states have become comfortable with providing basic Delaware law opinions, including the opinion required by Item 601(b)(5)(i).  The issue is not whether the lawyer giving the opinion is practicing law in Delaware.  Rather, it is whether the lawyer has the requisite familiarity with Delaware law to render the opinion competently.  See TriBar Opinion Committee, Third-Party "Closing" Opinions § 5.3 (1998).  As the TriBar Opinion Committee observed, "Mere admission to practice does not establish competency".   

The staff of the Securities and Exchange Commission agrees that "Counsel may opine on the laws of the states in which they are admitted to practice". SEC Staff Legal Bulletin No. 19 (CF) (Oct. 14, 2011).  Oddly, however, the staff will not allow lawyers to make the accurate disclosure  concerning where they are admitted (or not admitted):

For example, if counsel admitted to practice in Massachusetts writes a legality opinion under New York law for debt securities in which the indenture is governed by New York law, counsel may not indicate in the opinion that he or she is not admitted to practice in New York or is admitted to practice only in Massachusetts.

It is hard to explain why prohibiting accurate disclosure harms investors.  Perhaps the staff views this additional disclosure as a "hedge" by the opining counsel.  If that is the staff's view, however, that would be a reason to include, not exclude, the disclosure.  If the question is one of competency, disclosure of where counsel is, or is not, admitted is not material.  The staff should not prohibit accurate immaterial disclosures.