Opinion: extra cardio or sit ups

I usually have an extra 10 mins at the end of my weights sessions.the program i follow has me doing 3 sets of an abs exercise.

I have been working out for quite some time, but recently got injured. Coming back from my injury I noticed the more cardio I did the more my abs began to show (naturally logical I know), this was without doing a weights session in the gym or an ab workout.

Since my weights program is a German Body Comp one, ensuring that I am engaging my core, I wonder should I use that extra 10mins to go flat out on a treadmill or stepper to help shift those bits of fat in my trouble areas? (diet is good, i am being very critical of myself here in my obliques)

My cardio currently consists of football training, 5 a side and a 2 x 7km cycles per day (weekdays)

In short, do you really need to train abs?