National Security by definition incorporates a military role. Further, when one examines the elements of the portfolio, it makes sense that an individual from one of the branches of the armed forces be appointed to lead the office.
Not suggesting it isn't part of the portfolio, but I think the "makes sense" argument isn't that strong given that many countries around the world appoint non military/police persons to lead the ministerial portfolios which include policing and national defense. In Canada the Minister of Public Safety has responsibility for the RCMP (Police) and other agencies, and the Minister doesn't have a military background. The Minister of National Defence (where the military reports to) is also a non-military political head.
Granted we've had both Ministers of National Security with a military background (i.e. Alfonso, Griffith, Theodore) and without (Martin Joseph, Howard Chin Lee, Jack Warner, Emmanuel George, etc.) under both the PNM/UNC. My real judgement is on the plan, regardless of background - from what I've heard of Dillon, the emphasis he spoke of was largely what we've heard before (tools and weapons) versus some of the more boring reform that is drastically necessary.