An officer is not required to issue a citation for the alleged law violation that precipitated the initial stop. He merely needs to substantiate that allegation in his report in order to satisfy probable cause requirements. That is not to say that there may not be other reasons to believe you will be successful in the defense of your case. It is just that the officer's failure to issue you a speeding ticket will not affect the eventual outcome of your case.
An officer not issuing a ticket for the reason why they stopped you is not necessarily go in your favor. Usually an officer will testify that they thought a criminal charge of DUI was sufficient and did not find it necessary to overload the accused person with another headache. With that said the reason for the stop is important on whether you were "paced"- then there are very specific things the office has to do to make it be a valid "pace" to stop someone for speed. Or a stop based on radar - that the instruments were in working order need to be checked. These are just a couple reasons to view how a stop regarding speed can be properly challenged.
Unfortunately, no. The officer is not required to issue a ticket for the underlying infraction that led to the DUI investigation and ultimate arrest. He/she only needs to validate the reason for the stop in their report. This is actually a typical practice for many of the officers, especially troopers, who do not tend to 'pile on' by also citing the person for the infraction when they are going to forward charges for a criminal offense.