It's no secret I am a fan of using taps instead of switch SPAN ports when instrumenting networks. Two excellent posts explain the weakness of using SPAN ports and RSPAN.
Both of these were written by Tim O'Neill, an independent consultant.
This is the simplest way for me to compare SPAN ports to taps: a SPAN port is a girlfriend, but a tap is a wife. It takes a real level of institutional commitment to install a tap, and the rewards are long-lasting. A SPAN port is a temporary fling subject to break-up (i.e., deactivation).
Furthermore, I really liked the blog post's emphasis on SPAN configuration as a change that must be allowed by the change control board in any semi-mature IT shop. The only CCB action needed for a tap is the initial installation. Any change to a SPAN port configuration should be authorized by the CCB. This is one of the reasons why very mature (and well-funded) IT shops use matrix switches for on-demand visibility, as a mentioned last year in Notes on Net Optics Think Tank.