Quick workouts are better than no workout!
When you just can't make it to the gym, those 15 minutes you spend on your abs in front of the tv or on the front porch make a differnece not only psychologically, but also metabolically. You'll still burn a few calories while you're sleeping if you give your muscles a little challenge each day.
Keeping your muscles busy on the reg reminds your brain that a body in motion stays in motion. While it may not compare to DOMS in terms of feeling as if you've worked hard, at least your muscles aren't just sitting around doing nothing!
Make random workouts count with potent core moves such as V-sits or 15 minutes of dance cardio in your living room. (No one is watching, so have at it!)