Publishers should not sit on the electronic rights, hoping they will become valuable down the road. Electronic rights should not include posting or displaying an electronic version of the book, or excerpt from the book, but must include sales from which the author benefits. A publisher should not be allowed to use one author's book to promote other books. The last thing you want is for a publisher to hold on to the rights to a book just to keep selling a few electronic copies. Watch out for contract language that lets publishers not pay authors royalties for electronic rights.
Royalties based on net receipts have a way of turning into zero royalties for electronic rights.
Authors should be paid higher royalties for copies sold electronically, than for paper copies. Authors should try to retain control of electronic rights as much as possible. If you have a good reason for keeping electronic rights, such as you already have interest from an electronic book publisher, then you have a good reason to assert your right to keep electronic book rights. One way to get a jump start on protecting your rights is to register your book's title as a web site. Visit Network Solutions to quickly, easily, and inexpensively register a domain.