Only have Cable and ADSL here - so I just have one of each. Previously I had two ADSL lines (one on each of two phone lines) but speeds have increased so much that cable and adsl is a nice combo.
Sometimes I use MAX the ADSL for my uploading/downloading while using the cable for surfing etc.
Most important reason for having both though is in case one line goes down. I also have big UPS batter packs on each and on the wireless routers so I stay online in power outages
pretty cheap protection from being out of work for the day during problems