I've long been of the view that hands should precede kicks, especially the round kicks, because they do take much longer to reach a target then straight punches or some of the shorter, chopping kicks.
My preference for kicks will always depend on the positioning of my opponent. One of the things that I've learned from watching and talking to Dutch kickboxers, especially the ones who dedicate some time to making the transition to MMA, is that they are really effective at using the kicks based on the angles that they are engaging their opponents. This is something that isn't lost on the high level boxers, especially the ones who beat the snot out of me on a fairly regular basis now.
The lead leg kick can be very effective in situations where you are attacking an angle that is particular vulnerable. (The case I've found most valuable is that of a southpaw opponent who is standing a bit inside my lead foot. I've found that snapping the leg kick in against the thigh is a really effective way to interrupt balance that is very difficult to counter.) However, there are a lot of situations where kicks just aren't effective ways of opening up an angle of attack against an opponent.
Most very effective strikers will not make their legs vulnerable as a target off the get-go, and so it is often a very bad idea to just attempt to snap the leg kick out, as those who throw it as a round-kick (I don't use it as a round kick when I'm leading with it; like I said, I tend to snap it straight in on the angle, rather than bring it up at all) will often give up the better angle, especially on a counter punch. Against another orthodox fighter who leads with the lead-leg round kick, there's a lot of vulnerability created to a lead left hook. More generally, that lead hook is one of my favorite ways to start combinations in an MMA setting against anyone who likes kicks with their lead leg, because of how well it takes over the angle.
One of the things that I'm continuing to gain an appreciation of in my boxing training is the jockying for a superior angle that comes about among professional boxers. It is a lot like the grip fighting that obsesses a lot of high level judo and jiu-jitsu players. I've never really understood the purpose of a lot of the set-up head movements used by a lot of boxers until I've started to realize that it's about trying to find an effective angle for that straight punch or upper-cut that you want to fire off.
You can look for those angles with kicks, and certainly a lot of high level fighters are very effective at finding them, but that is not my preferred approach, and I think I'm definitely not alone in thinking of kicks as a generally sub-optimal way of dealing with opening up a combination.
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