Why use vacuum tubes today?
The short answer is when used in audio circuits they just sound better than transistors.
These is quite a sweeping statement, but consider that if they did not have a considerable effect in the audio industry why after 85 years since their introduction we still use them today? Also consider that musicians (who should care) generally prefer tube amplifiers and tube microphones, and recording/mastering studios still feature many tube based components. A very strong argument indeed; tube products may not be the choice of the mass market but they are the choice of the discerning music lover and are still very much in use today.
We believe the true emotion of music and the feeling of 'realism' is enhanced by tube products. This is chiefly because they are low gain devices which have a very linear response, even without feedback. Tubes have an even-order harmonic response whereas transistors have odd-order harmonic response, which is why it is important for all transistor amplifiers to have very low harmonic distortion, as an odd-order response makes the sound very hard and sterile. That is why solid-state designs keep emphasizing the 0.0000000% THD measurements! Solid-state designs also implement huge amounts of feedback in the order of 50dB+, whereas tube designs are typically 10-15 dB.