GP's must maintain a hygiene dept in order to fill his/her schedule. during hygiene checks, fills are the most common dental procedure found. And the gp must provide this service to his pt's or the pt will simply go elsewhere. So, its not necessarily his choice, he must provide general dentistry to his pt's as he is so advertised.
Imagine going to your family dentist and being referred out for a filling!
GP's , however, do seem to specialize in certain niches though they cant legally advertise as such a specialist. IE, one guy I worked with did only crown/bridge and fills and referred out 90% of his endo/OS -> he did quite well infact. So, as a gp you can have a larger % of your procedures in on specialty, but again your title almost requires you to provide the general dental care to your patients. Many gp's are very capable in their niche, some would argue as capable as specialists.
Specialists, on the contrary, do not require hygienists (exp perio) and rely on referring dentists for their work. They are not required to provide general care and are only referred procedures within their specialty. Thus their schedules are more easily filled with only high producing procedures.
Additionally, you find it rare that a GP will refer to another GP if a specialist is available in the area making it nearly impossible for a GP to perform ONLY large production specialty-type cases.
to those with more experience - does this sound accurate?