Brain-computer interfaces (BCIs) offer a direct channel for communication between external devices and the user's central nervous system, holding significant promise for restoring limb mobility in patients, particularly those afflicted by conditions such as peripheral nerve injury. However, the widespread utilization of BCIs is impeded by the absence of an objective and user-centric evaluation system. This paper proposes a novel BCI evaluation index termed "brain resource occupancy," serving as a quantitative measure of an individual's cognitive resource utilization at any given moment. Derived from cognitive psychology, this metric is perceived as a crucial determinant of performance across various single-task and multi-task scenarios. The paper delves into the concept of brain resource occupancy, its significance, measurement techniques, and underscores its potential to objectively assess BCI system performance from the user's viewpoint. Additionally, it explores the correlation between brain resource occupancy and the privacy protection and security facets within the realm of BCIs