The primary advantage to grafted roses is for the whosale grower, it allows
them to get a plant to the market place a year earlier than own root plants.
The root structure of a grafted rose is already established when the
grafting is done; this gives the grafted portion of the plant a boost in
establishing new canes.

Root stock is usually selected to perform in the climate and soil conditions
of the whosale grower. The root stock may not perform well in your
location. For example the growers in the SE area of the US use fortuniania
because it does well in the region. Those in the NW usually choose Dr.
Huey.

Some varieties don't do well on their own roots, the breeding program was
based on foliar and bloom performance based upon grafting. Little
consideration was given to the root system of the hybrid and as a result
they may not have a healthy vigorous root system.

Another reason for grafting is to prevent suckering, some old varieties and
species roses have VERY vigorous root systems that sucker readily and can be
a nuisance if they are not grafted. Austrian Copper and Persian Yellow come
to mind. Keep in mind that the primary goal of any organism is survival and
propagation, the species roses were successful because they could perpetuate
them selves, often due to a very robust and vigorous root system. Think
about some of our 'pest' plants, you can cut all the above ground growth and
they return from the root system. Kudzu in the South East US and bind weed
in the west are prime examples.

Just a few thoughts about grafting and roots.

Tim