It is not always worth doing a major restoration on a piano that was not of a high quality to begin with, however even a lesser piano may be revived by tuning and repairs to extend its life until a better instrument can be found.
Not all pianos are worthy of restoration; many lesser-known makes/models are not sturdy enough or of adequate quality to justify the investment. If you have a high quality piano such as Steinway, Blüthner, Bechstein, Bösendorfer, Richard Lipp & Sohn, Feurich, August Förster, and Rönisch then restoration would certainly be advisable and usually a worthy investment.