Once the war started, I could see the GEF getting away w/ something like that, and once the UEF is busy fighting the Tevs, the GEFs would have more freedom in using something like a cruiser. But before the war, even if the GEFs could basically acquire a cruiser for free, I don't think it would be worth the resources and manpower to keep it operational. The GEF were litterally the only ones around for the UEF to fight, so you can bet that once the UEF realized the GEFs had deployed a cruiser in an engagement, they would send everything they had after it. Considering the size of the UEF fleet, and the fact that before the war it had nothing to do but training exercises and hunting GEFs, there would be no way a couple of cruisers would survive once the UEF had their location.