IMHO - The worship leader is the Senior pastor
The guy that leads the music is the music leader.
WHY - because worship is much more than JUST music!
Actually, I believe the term "Worship Leader" is a fluidic position if you really think about it.
Shouldn't the leading of worship be synonymous with "Instigating" worship? How then do we "instigate" (provoke) worship? Consider these examples:
When someone gives a testimony or "praise report"
When someone prays audibly and publicly and what they are praying strikes to the heart of those who are praying with them
When someone sings a "Special" and it stirs something within you causing you to raise your hands to the Lord
When a message is being preached (Bible study, Sunday School, Devotional, etc.) and the Holy Spirit stirs and convicts hearts, call to action, etc.
The pastor is most certainly a "Spiritual Leader" of the congregation and as a leader, he will certainly "lead" worship at times