Like boink, we still have our 'normal' phone for people to phone in (especially our kids who get free calls on their mobile contacts). This also has the answerphone - no real need on the broadband one? The 'normal' phone is also used for those calls that you have to pay for (0845? and premium competition lines etc.).
We use the broadband phone for calling out - you get free calls at evenings and weekends but we took out the extra for daytime free calls - only �5 per month. You do get an extra number (ours is quite weird, like a mobile, but a friend managed to get a local STD code on his number).
It is handy having the two phones because if one person is 'hogging' the phone, you can always use the other one!