I don't think that there is any disadvantage to using a common resevoir.
I agree that it would be easier to change the slave cylinder if it were outside. However, the geometry of the clutch release linkage is better with the slave inside. The throwout bearing is applied more evenly than if it were actuated with a fork on a pivot. Fortunately, our cars don't have much trouble with the slave cylinder.
The biggest enemy of the slave cylinder is cruddy brake fluid. Change it every two or three years and it will last a long time. I change mine annually.
