Intensity during base

Even though Xert recommends tempo and endurance rides during the base period (for athlete type sprint time trialist), is it also a good idea to do a shorter but more intense ride (like the pure pursuiter) to train higher energy systems 1 or two days (during base)?

Yes. Adding some intensity does mix things up. Although the benefits of higher intensity training dissipate quicker, the added strain introduced on your lower intensity from the high intensity work, particularly for longer workouts, can still be very valuable to your progress.