It seems mandatory that you include an advisor in your command squad, these days. I never liked the Master of Ordnance since he would force me to keep my unit static. I play with a mechanised army so I rely on mobility. The Astropath could be very good in certain builds but since one of my armies assets is a strong Alpha Strike, I don't like to put anything in reserve. So including an Officer of the Fleet seemed like a good idea when I was building my list.
I mean, what's not to like? For a mere 30 points you get the chance of seriously disrupting your opponents plans. With a -1 to reserve rolls and forcing re-rolling successful outflanking rolls it can hold his units back for several turns. This can be very handy against certain type of armies (Daemons I am looking at you) since it allows you to focus your shooting on the units that are one the table and when enemy reserves arrive piecemeal you can move on shooting them.
If everything looking fine on paper what's wrong with him then you might wonder? Unfortunately I made a shocking realisation during my recent games. Officer of the fleet can cost you the match. A smart opponent will use his ability against you and he could steal the game like so. Let's assume you are playing an objectives based mission and you win the roll for first turn. After you have finished deploying, your opponent will realise that placing his units on the board is not the best of ideas. With the amount of shooting an Imperial Guard army has, you could kill half of his units with a good round of shooting. He decides to keep everything in reserves. If he had ten reserve rolls to make (and taking the Officers ability into account) he will get on average three units on the second turn, four on the third , two on the fourth and one on the fifth. But what does that mean for you?
First of all you won't shoot at anything for 2 turns. Since the Guard relies on shooting stuff to take them out this is obviously a bad thing. For the reserves that arrive on turn two and three you'll get 2-3 turns of shooting which is good enough. Still, with all that firepower it is going to be overkill and most of your shots will be wasted. But you may not have the time to shoot at the unit that arrives on turn 5 at all. So the opponent will have a very good chance of reaching his objectives on the last turn, and if he is lucky enough to have a unit deep striking he could also cheekily deny your objective on the last turn. This almost happened to me last week.
Plus most of my mobile, scoring units are not very good at holding objectives. If I am close to the opponents table edge (where his objectives are going to be) he will have a good chance of destroying my unit when coming from reserves. A chimera with 10 guardsmen can be easily neutralised by any MEQ unit. So even if I am on his objectives on the last turns no one can guarantee that I am going to stay there until the end to contest it. I have to destoy his units beforehand but if they are not on the table, I cannot. Another minor point is that with 30 points that he costs you can actually buy something that can kill some stuff.
So in a nutshell, the Officer make your games unpredictable. You can't be sure when the opponents reserves are going to be arrive and combined with an early game finish can make you draw or even worse lose. Furthermore it means that you are going to be shooting at less units for less turns and that's not something you want with a very shooty army like IG. I am personally thinking of dropping him out and with his points get Demolitions for one of my Veterans Squad.