That kind of food can maintain the human body for small periods of time, but to sustain you for longer periods you need fibres. Powder food doesn´t have any. You can artificially induce tablets with proteins, sugars, and vitamins, but you need fibres, hydrocarbons, and other stuff only found in meat and vegetables. There is no way around it.
There is also one key factor that everybody forgot to take into account: moral.
You try to feed 5000 men with tablets and dehidrated food for a year or so, and you have a mutiny on your hands! If any of you had ever spent time cucked up in a ship, surrounded by guys 24 hours a day, and with no land in sight for months on end, you would know just how much the meals are important to the crew of any vessel. Trust me, i´ve been there. That´s why they feed you so well aboard a battle ship. Food is important to keep moral up. Feed them ****, and you have 5000 grumpy men with zero will to fight. That´s just asking for trouble.
Of course, you could always feed them Prozac, mixed with all the other food tablets, to make them not care... But then again, you wouldn´t want 5000 zombies as a crew when the time came to face a fleet of Saths coming at you.
Wich is actually true in a sense, the US Army is feeding chopper pilots and tank crews with "speed" and other stymulants in Iraq, to keep them awake longer and more alert. They got so strung up on that crap that they can´t tell who´s a friend or who´s the enemy anymore, hence the huge number of friendly fire incidents.