First off, ground and local should not be combined. That is something that the FAA has been complained about many, many times. You will find this happening on the midnight shifts when they go down in staffing for the night. However, this should be the only time.
There are SOPS ( Standard Operating Procedures ) that do state to keep people on the appropriate frequencies. It makes it easier to split off the positions, but it also keeps the frequency spectrum as it is supposed to be used. Shouldn't be using a ground frequency in the air due to the range of use limitiations. You are going to possibly be bleeding over to someone elses's ground freq, if you extend the range while airborne.
There is a way that you can keep from getting stepped on and that is mechanical. Radios can be set up to rebroadcast from other frequencies. The Center equipment was set up to do so, however, it would have cost more money to pair all the frequencies to allow this to happen. So we don't. The radios that the towers have could be made to do this if the capability were added to it, again at an additional cost.
As to enroute frequencies and using the same ones. In areas that we can do this, we do for the very reason that you cite. However, due to the large volumes of airspace some of our sectors have, when we combine up at night or even in the day in some areas, we can not get appropriate coverage at the low altitudes as well as some of the high altitudes.
regards
Scott