Rosewood is a soft and very poreus wood, which is why it's commonly only used on fretbaords and acoustic backs and sides.
Rosewood may be used on backs and sides and fretboards, but not necessarily because it is soft and porous. Usually fretboards and backs/sides are made of harder and denser woods such as maple, ebony, mahogany, ovankol, etc. If a soft and porous wood was ideal for fretboards and backs, then we'd be seeing more made of alder, poplar, basswood, etc. If you use a piece of wood that is soft for a fretboard, it not withstand playing wear or holding in frets. If you use a soft wood for the back of an acoustic, it won't project as well. Soft woods such as spruce are used for the tops, and I believe there are some acoustic guitars out there with rosewood tops.
Rosewood varies in softness, density, and porousness. But it is an oily wood. Rosewood is mainly a family of woods that also includes Pau Ferro, Morado, Indian Rosewood, etc. All these woods can vary in terms of density and hardness, from one family to the next, and one piece to the next. The rosewood that is used on fingerboards and backs of premium instruments tend to be a harder rosewood type, or if they use something cheaper it may be a laminate.
