Kickboxing makes able to hit hard&fast, and to be able take a punch without freezing up -which is very necessary for self defence. But that said, it also teaches some terribly bad habits for self defence.
So do all fight-sports.
A self defence situation is not a fair fight, starting at a given command at safe distance with two fighter circling each other looking for an opening. It does not involve feints (and rarely blocks), in/out movements or long (30-40second) fights. Fights are what happens when self defence fails.
Self defence is all about "get free, hit hard enough to buy you time, get away FAST, dont get trapped in a dick measuring contest".
Self defence situations (once self awareness and avoidance fails) are fast, brutal and are over in 3-4 seconds, or you are in trouble.
Typical situations are an opponent are close in and demands something from you (possibly grabbing you so you are unable to get away), who grabs you (or pushes you against a wall or something) and starts to punch you. screams and bull-rushes you, or (especially in the case of women) grabs you and tries to drag you somewhere.
Grappling fight-sports always focus on getting the opponent down and holding him there. Which is fine if you are one-to-one, with the enemy not palming a knife (the nature of grappling makes a knife a very nasty surprise) or having friends willing to kick you in the head while you are down on the ground pinning the first enemy. And if you ARE one-on-one with an unarmed enemy, you better hope your friends get there in time or that you are able/willing to choke him or break a limb to immobilize him. Sooner or later you will have to let go, and then he will right there next to you.
Self defence training courses (krav maga etc) usually gets the tactics right, but fail to toughen the trainee to be able to take a hit, or give him the ability to hit a resisting target.
Three interesting podcast related to the subject:
Streetfighting: Why streetfights are not self defence.
http://iainabernethy.co.uk/sites/default/files/podcasts/Street_fighting_pc_10_13.mp3
In defense of combat sport: What TMA/self defence NEEDS to learn from sport fighting.
http://iainabernethy.co.uk/sites/default/files/podcasts/In_defence_of_sport_04_14.mp3
The martial map: martial art, self defence, fighting -why they do not necessarily overlap.
http://iainabernethy.co.uk/sites/default/files/podcasts/The_Martial_Map_Jan_11.mp3