His x-ray doesn't scale well with combos, personally I feel it's a waste of meter in said scenario. Generally it's used in reaction to projectiles and jump ins. While Reptile can't do upwards of 50% combos like Kitana can per say, his combos are pretty consistently around 34-37% damage without meter; which I feel is pretty good. I believe his x-ray does around 36% without scaling. I personally feel his x-ray is situational. Meter should be reserved for EX slow FB pressure and breakers imo.
However, you can use FBs to your advantage due to them being unbreakable - while the scaling with the x-ray is minuscule it does guarantee a safe setup. Certainly an option if you're both low on health, you've got full meter, and your opponent has a breaker.