Rubato has always been related to musical performance expressivity. In the last 150 years, however, Western art music teaching has emphasized strict metric regularity in the performance of pieces composed before the Romantic era. Such practice however is not consensual. The present study is an interdisciplinary, descriptive analysis, based on musicological, psychological, and neuroscientific data that aims to explain the reasons of the polemic related to the function of maintaining strict metric regularity in musical performance. We have found in cognitive psychological data, information that supports the relationship between rubato and musical expressiveness. There is also evidence that the maintenance of strict metric regularity could inhibit expressiveness. This inhibition seems to be related to the level of difficulty that one experiences when asked to keep a strict tempo. Despite the inhibiting component, observations in musical teaching indicate that the practice of strict metric regularity is an indispensable instrument to a more elaborated expressive musical interpretation. Therefore this present study analyzes the importance of this practice in the development of musical expressiveness.