Visual cues, particularly triangulation with two sight lines (although more difficult in line abreast) are THE key to good station keeping.
A proximity sensor will just add a level of complexity and distract the pilot from what he or she should be doing, i.e. scanning the cues and continually adjusting. The only time I can see some artificial system being any good is if it was extremely accurate and fast acting, and linked to an autopilot system that would do the flying for you, which rather defeats the purpose!