how important is it to use crossovers when adding subwoofers to a system?

esipilka, it all depends on your speakers. If the Sacha woofers to not go up that high there is not as much advantage in using a crossover. The Sacha probably starts rolling off at 40 Hz or so in most rooms so you can just put the subwoofer under the Sacha and make it 3 or 6 db down at 40 Hz and adjust the volume and phase to match. Using a crossover to get the bass out of the speakers is much more important with full range speakers like ESLs and to a lesser extent planar magnetics. If a dynamic speaker's woofer go up over 1000 Hz then there might be a benefit particularly if you don't have a lot of power in your main amp. You do have an excellent analog cross over in your woofers. The only thing superior is digital bass management. There is one other consideration. If you like pushing the bass louder it is better to run the subs up to 100 Hz. Then you really should use a cross over.  You can then independently push the volume on the subs. This is very useful listening at lower volumes because our ears sensitivity to bass drops dramatically. See Fletcher - Munson. Get some extra low level cables and play around. That is what makes this hobby fun. Try it both ways. Once you have things hooked up it is just a matter of twisting knobs and punching buttons. 

One of the main ways a sub improves bass response is by adding additional bass locations. This is the foundation on which a distributed bass array is built. The best solution to the problem of getting really good deep articulate bass in a room. In essence, the more subs the better. If the two main speakers are used full range they continue to serve as additional bass sources. So whatever is in theory gained by using a crossover is lost by losing the two extra sources of bass.

Running subs additional without crossover entirely avoids putting another component in the signal path. Running without crossover retains the original pair as bass sources. Its totally the way to go.

It depends on the speaker and the amplifier. Examples - If you’re using a low powered Class A amp on the speakers, it might be a good idea to use a crossover to create more head room for the amp. Or, if you are using small speakers that don’t go very low, like BBC LS3/5As, an electronic crossover can use the small speakers like midrange/tweeter units and move the crossover point high enough to reduce distortion in the speakers.

With your Wilson speakers, I don’t think either of those cases is applicable, and you can just use the subwoofer to fill in at the bottom.