The sub will still have some info above the fundamental frequency. In order to create a rich sub you need to have some harmonics IMO. These harmonics may interfere with your kick. Also, depending on the slope you cut your kick at, there will still be some lower frequencies coming through on your kick drum, which may interfere. Side chaining isn't essential. I think people just do it because it is a nifty little production trick. It can help you gain a louder mix but when done wrong it can make your sub sound shit. It is a technique that is often overused. Leave it as a last resort. Get your balance and eq right first
yeah sam's right..... People over think side chain compression on the subs and kick, its used mainly in house and commercial dance tunes.
You dont need it if you get the kick sitting nicely on a sub and they dont clash, add a bit of attack to the sub if it hits at the same point as the kick..... The sub doesnt have to hit at the same point as a kick btw, sometimes in my tracks i have the sub hit a bar after the kick, even on the drop. sounds fat if done right.
Whether you need to side chain depends on several things, there are no set rules about it, personally I always side chain, because I like the pumping sound you get as the kick and sub rumble along, but also for technical reasons, ducking the sub (thats the technical term) by side-chaining the kick, means that you have maximum headroom for the kick, rather than having it fight with the sub for volume
Try it for yourself, setup a simple sine wave synth, low frequency sub style, no harmonics, and play it as a continuous note...then try playing a kick over the top, compare the sound of the kick when played without the sub, you will hear that the kick is weak, you may even notice that the sound as a whole becomes muddy.
I like big fat kicks, not all of drum and bass is made with kicks in this style, but as already said, the more harmonics you have, and the more crossover you have between the kick and sub, the more you need to either side-chain, or EQ; over doing EQ can kill the sound, side-chain allows for a richer sound, without worrying about headroom in the bottom end
I think it's not really necessary unless your mixdown sounds really muddy and your frequencies are overlapping
It depends on the track how you want to make it sound and stuff but that's just my opinion