if I were only in sandy or mud bottom, then one type of anchor (danforth) would work all the time, but I never know what the next anchorage will have so I carry both a danforth and a new gen style (rockna)....
when we were at lake tahoe last summer, we were caught on the east side of the lake during a severe storm.
anyone being familiar with lake tahoe knows there is no shelter on the east side of the lake, and it has a 10-11 mile fetch for the wind and waves to build in.....
it was evening and rather than sailing or motoring the 11 miles into the waves to the sheltered west side of the lake we decided to anchor up near the lee shore and hope for the best.... we were in 15ft of water and I used the rockna thinking it would hold better, but the bottom on that side of the lake is mostly waterlogged pumice rather than sand.
the swinging of the boat in the wind caused the rockna to just walk down wind as it was buried on the bottom (the water was clear and you could see the trails left by the anchor as it pulled thru the bottom), so after 3 attempts I tried the danforth, and it anchored us solid.
about an hour later I decided to run up and throw the rockna out so we could hang in a V on both anchors to stop the swinging, as I was afraid to go to bed trusting to one anchor and rode in the 30mph wind and 4ft swells.... they kept us safe thruout the storm
so even though the rockna didnt do us much good by itself in that type of bottom, its better in a clay bottom than the danforth would ever be.... and no matter what anchor you carry, if the bottom is foul, any anchor will hold.. but you may lose it to whatever is on the bottom.
personally, I see little sense in carrying 2 anchors of the same type, unless you have others onboard to choose from as well... but it has been proven to me that the new gen anchors arent the best for all types of bottoms all the time..
and I also believe the stern rode needs to be longer than the one at the bow, as there will be times when you may want to use it as a stern tie to shore....