@lanx0003 , phusis hit the nail on the head. The benefit of using a two way crossover increases the wider the frequency response of the main speaker's woofer. Speakers with a large woofer that only covers the bottom 250 Hz are not going to benefit as much as speakers with smaller woofers that cover the bottom 1000 Hz. This includes a big chunk of the midrange where the decrease in distortion is very noticeable. With full range loudspeakers like my Sound Labs IMHO it is mandatory. You turn a polite speaker into a disco rocker. There are other advantages. If you are using digital bass management which includes the crossover you can place the subwoofers where they work best and are most efficient (in corners and against a wall) and correct them so they match the mains speakers in time and phase. If you like to feel the kick of a bass drum you can consider this mandatory. If you have a low power main amplifier like a SET this is also mandatory IMHO. You will increase your headroom and keep the SET away from clipping.
The manufacturers know that most people are cost and complexity sensitive. Using just a low pass filter and pulling the sub in under the main speakers is an inexpensive, easy way of running a sub. If you have to start out this way as a temporary measure to get yourself into the game, go for it. Down the line get a preamp with bass management or a crossover like the MiniDSP and you are really in business. There is a tendency to dis digital equipment but IMHO the benefits far outweigh the disadvantages. There is so much you can do in the digital world that is impossible in the analog world. My days of being a purist are long over and I never plan to look back. Being able to tune your system exactly the way you want it is a huge advantage. The trick is to keep everything digital until the DACs before the amplifiers. This means you have to digitize your phono stage but if you do it in 24/192 the conversion is invisible.