Coming from a profession where physical fittness means life and death (U.S. Army Infantry), I can truly say that IMO ab workouts are a nessessity. Without a strong core (lower back, and abdomen) the rest of your workouts will definitly suffer. Yes, all of the different squats use tha abs and so do a lot of the free weights; but they only help out so much. Doing Ab-focused exercises help with the stabilizer muscles, and strengthen the core, which in turn allow the body to be able to handle a little extra weight, or the same weight for longer periods of time.It will take a few weeks for your muscles to adapt to regular exercise - they're going to start to contain more water in order to be able to store more nutrients, which means they'll get a little bigger relatively quickly. Also, don't bother checking your weight at this point, or the body fat ratio.
As for ab exercises, I personally don't really believe in them... The main purpose of the lower torso's musculature is to stabilize the lower back when trying to exert any kind of force with the upper body. This means that if you're working with free weights, it will get exercised as much as it has to. Squats, front squats, standing presses, deadlifts, basically any kind of free weight exercises will all use it, doing extra sit-ups and crunches is not necessary. I've never really done such focused exercises and still my torso muscles developed a lot; although ab wheels are an interesting piece of equipment that we did use during circuit training.
Oh, and this topic shouldn't be frowned upon here, IMHO. Spartans are strong and healthy and everyone will look better in their armor if they aim for a good physique, too. Wouldn't like to offend anyone, but both underweight and overweight Spartans look a bit wrong... and this is something everyone can change, for the better.