Sure, you can do the regular routine of Saturday long ride, Sunday long run, but what if there's a better way? Most athletes follow this schedule, but does that mean it is the most effective? Plenty of people do things which aren't necessarily the most effective, but is popular enough to bring comfort to the decision.
What if a 10 day cycle worked better? Or 14 days? Or what if you simply did 2 days in a row of biking, and moved your long run to the middle of the week?
Think about your training and performance. If you've hit a plateau, and aren't seeing the steady gains, maybe it's time to try a new routine. Change it up! Plenty of options, as your mind and creativeness, (or your coach's), are the only things holding you back.
If everyone else is doing the same thing, how do you expect to get ahead?