My experience with volunteer soloists has been mostly negative. It creates competition among members ([our] choir is very social), frequently provides poor quality music (some of the worst solo singers will volunteer), adds absence risk on the performance night and leads to part ownership ("[So-and-so] always does that part"). I am a supporter of an ensemble solo strategy that, while more difficult to rehearse, solves all of these problems with multiple voices for each solo.
I don't feel myself that all people who want to volunteer for a solo should have one, but I do feel that those who are capable and have that gift of voice to be able to do a solo should be able to contribute a solo on a regular basis. Member singers are people too, with desires and creative ability and ability to inspire and bring a special feeling to a prayer or song. Competition is not a bad thing, necessarily, unless one believes in the socialist, communist point of view where individualism is not a good thing. I feel it is a positive for volunteers, who are able, to be in a position of leadership as well as growth, and having an occasional solo (and I don't mean just getting up to do a performance piece but an integrated part--a line in a prayer, or part of a song, or a particular prayer, etc) is an opportunity to be a leader in a way that a singer can be one. Volunteers who come every week to sing at a service, give of themselves and their time, also need some kind of encouragement and opportunity, growth, and variety. I believe that many congregants are proud of their members who can participate this way and many appreciate the talent and abilities in their congregation and they do not feel it is a performance for a volunteer member to have a solo here and there. What causes more controversy is when hired outside professionals are brought in to often do what some of the volunteer singers are capable of doing. If this is not a performance, then I say use the talent that is available in the congregation whenever possible. The best cantor and cantor soloists and rabbis I have been privileged to sing with or learn from were those whose egos allowed their choir members to have an occasional solo rather than take all the credit for themselves.