One problem with such volunteer jurors is that they are not what the judge and the lawyers want to see in the box. These people are probably retired, well off, competent, and are ready and able to be jurors ... that's exactly opposite to what the professionals in the courtroom want. The judge wants jurors that are easy; best if they know nothing at all because then they accept anything that lawyers say as truth. The recent trial(s) of Jammie Thomas are quite representative in this aspect because the jurors spoke after the trial and demonstrated, for all to see, their complete lack of clue.
Multiple juror experiences of such volunteers would also educate them beyond what is desirable. They'd become semi-professionals who are actually aware of their rights and not always willing to reject common sense.
In other words, jurors are like politicians - "if you want the job then you shouldn't get it." A juror has certain power over people; if chosen randomly and rarely, this can't be exploited. But if your neighbor is spending his time in courts day after day he becomes a somewhat important person, without being elected or hired by the parties.
It would depend on the venue in question. My experience with volunteer jurors is that they are often neither retired nor well-off, but rather tend to be homeless panhandlers who supplement their daily "will work for food" activities with the occasional meal provided by the court. I suspect that is true in most urban areas.