when they are producing sacred music they are undeniably PRAYING at the same time.

It denotes a 'musician.' To me, 'musician' in this context is a professional designation, like 'doctor' or 'lawyer' ('teacher' etc...) which assumes professional level training.

weekend warrior

IIRC that term was originally applied to members of the US Army Reserve and Army Reserve National Guard, which members met one weekend/month to drill (and also had a 2-week summer camp obligation).

So as a purely practical matter, and assuming we can't actually peer into folks' souls: would you rather have your music program run by a) a professional who is an experienced organist and choirmaster, familiar with repertoire and techniques and capable of raising standards to the highest possible level the practical situation will allow (often higher than anyone thinks!) or b) a demonstrably devout volunteer who may lack the skills and experience to accomplish the above?

...to limit liturgical singing to only the best singers & musicians seem to me to miss the point.

b) a demonstrably devout volunteer who may lack the skills and experience to accomplish the above?

Well put. The sad reality is that many priests would rather hire the person described by B.

• Church musician = music professional?
  I appreciate the distinction that Mark and others have written regarding a musician as one who - usually by formal, degreed training - is a) professional and b) expert... and a "church musician" therefore as the expert professional musician who spends all or part of his/her life in serving the church via music.
  
  I imagine that like most of us, I have seen both "trained musicians" and "amateur musicians" who had no ability or business to lead a parish choir; as well as "amateur musicians" who were knowledgeable and highly skilled and able to lead (perhaps, truth be told, better than some "professionals"); and of course the anomaly: those "professional trained musicians" who weren't very professional, even if they were knowledgeable.
  
  Is the volunteer who spends 4-5 hours rehearsing every week; another 1.5-2.5 hours singing Liturgy every week; who is capable and proficient in both chant and polyphony not to be considered a "church musician"? I've been a member of parish choirs that were undisciplined and perhaps unworthy of carrying the title... but I've also been part of groups that dedicate themselves completely - that take voice lessons on their own time, that study and learn outside of rehearsal. Perhaps the concept is more amorphous than being tied to a degree or even a line of work, just as it should not be simply connected to enthusiasm or desire.
• Singing vs. praying?
  I think one of the reasons "He who sings, prays twice" is because of the act of charity that is being performed - charity toward one's neighbor. The temptation for some choir members is to think that when we are bogged down with keeping straight what's happening next, making sure we remember to sing this particular section correctly, focusing on technique rather than substance - that somehow we are not praying. I would suggest, rather, that our prayer is one of Martha, not of Mary - that we are active in how we pray, not contemplative.
  
  Our charity is not simply that we offer our mean gift to God, but that we sacrifice how we might CHOOSE to pray if left to our own devices... in order to better facilitate the prayer of those in the congregation, in the sanctuary, perhaps even in the loft. That, hopefully, we elevate and inspire the thoughts and prayers of those in attendance to better increase the honor and glory of God.
  
  When I'm not in the loft, I generally don't sing - it isn't the way I necessarily choose to pray on my own. Instead, I take the opportunity to contemplate without having to focus on "what's next" or "must remember to avoid this mistake".