When playing the violin, students these days are often told to avoid portamenti altogether, or to only slide occasionally. Portamenti are often regarded as distasteful, self-indulgent, or dirty. Extremely sterile playing is regarded as a virtue, especially in an orchestral setting. Clara Rockmore's theremin playing has made me rethink the use of portamenti in my own violin playing. After all, the violin has no frets. Before Spohr, and the invention of the chinrest, the portamento (slide) was a much more necessary aspect of violin playing. After much thought on the subject, and reading Ruggiero Ricci's treatise on the subject (Ricci on Glissando: The Shortcut to Violin Technique), I firmly believe that portamenti (glissandi), should play a more commanding role in violin technique. As a side note, some musicians make an effort do differentiate between portamento and glissando, though physically, the left hand action is identical. Should we sound like cats mewing? Hopefully not. Portamenti should sound "vocal." Also, a delicate glissando can be used as a vehicle for great listening and great accuracy, especially in double stops and in virtuosic passages.
My favorite Clara Rockmore video is the "Hebrew Melody" by Achron, a 20th century composer know for his film music. The portamenti in this piece sound weeping and full of sorrow.