Low kicks aren't just about compromising an opponent's movement/ability to launch attacks or setting up you own shots by 'mixing it up'.
They can also be used to fuck up their offensive rhythm. Every time the opponent throws his hands, he is stepping forward and putting weight on the lead leg. If you are skilled enough, you can catch him then - doing damage and throwing him off-balance. In addition to this, a leg that is stepping forward or planted cannot be raised to check the incoming kick.
This is why I don't believe in scoring fights based strictly off damage inflicted by individual strikes. Defeating the opponent's offence, for ex., contributes greatly to giving yourself the opportunity to finish the fight. You might finish most fights with strikes to the head, but what got you there might very well be the low kick, or the jab, or the knee to the body.
By the way: I see a lot of MMA guys getting countered with punches as they throw their low kick. This happens because a) their timing is poor, b) they are throwing weak, non-commital kicks that don't knock the opponent off-balance and c) they are not using the correct defensive posture and movement (angling out, leaning back, moving the head off-line)