Ayn Rand took social security when she became eligible for it. That didn't make her a hypocrite - she never advocated in support of having the money taken from anyone. But once it was on the table, what was she to do? The goods were already stolen, and her only options were to take the goods or leave them to be consumed by the thief in other ways.
I view military service in a similar manner. If you live in a statist society, the sad fact is that you have to take advantage of what benefits you can while you can, or those already-stolen resources will simply go to other ends. Claiming resources which have already been stolen and cannot be returned is no more supporting statism than eating roadkill makes you a hunter. For a lot of people in these societies, short military enlistments lasting two to six years are simply the most beneficial job offerings they will get compared to other options on the table at the same time. To the person who enlists, takes the benefits, uses them to develop marketable job skills for themselves, and moves into the free market, I hold no judgment. They had to do what they had to do, and that meant taking the most individually beneficial option on the table for them at the time.
Libertarians and anarchists need to come to terms with the fact that they live in statist societies, and cannot arrange their lives as if they live in free societies when they do not. To do so is to be a vegan who thinks they can reduce the demand for animal products by not buying any, all the while the state subsidizes the ranching and factory farming industries. In that environment, your individual refusal to purchase subsidized products means nothing.