Front Speakers in a 7.1

[bz1080p]

This may be a stupid question but what benefit is there to having front speakers with frequency responses below 50-60 Hz if the cross overs are typically set at higher frequencies? (like THX @ 80) and having the subs handle everything from there down.
I see some people that have full blown towers that get below 35 Hz or or even lower?
It would seem that good sub(s) and a good set of sats. would give you everything you need for HT and music for the most part?
Thanks, BZB