Here is where you are missing something. Your mains don't do a lot of base, but they try to. So you still can get excessive excursion that will cause distortion in the range they do produce. So a HP to the mains reduces distortion and protects the woofers. If you want to investigate this further, download WinISD and model a woofer. Any one. Look to see the cone excursion in a simulation. Might scare the heck out of you. For this same reason, I am a proponent of sub-sonic filters. For music, a first order @ 20 to 25 works, but if into HT effects, might want a higher order just a tad lower. FWIW, I run a 24dB @ 60 Hz between my mains and sub, a 20 Hz first on the system. The difference is audible. Not great but audible especially if you crank it up to orchestra levels.