What bothers me most about this thread is that the tone is so accusatory and designed to put people on the defensive, rather than initiating an honest discussion about what can be improved and how. Let's just get a couple things out of the way:
1) Every owner of a rental home that I have ever met is very concerned about their homes being kept clean and wishes to provide his or her guests with a positive experience.
2) Every management company that wishes to stay in business knows that housekeeping is the most important part of their job.
SWGB makes some excellent points about the logistics of washing comforters on site after every guest. It is not possible. Not only does the housekeeping staff not have the time to do it in the house on any given Saturday, as he points out the comforters could not be properly "sanitized" to the same standards as the sheets and towels.
Many management companies use a linen pool where linens -- all white, cotton, and sanitized according to standards -- are intermixed among houses. I would venture to guess this is the way it is done most of the time. The only way to achieve the level of sanitation that you desire would be for companies to require all of its houses to use the same pool of comforters -- probably white matelasse covers or blankets because you can't force all owners to upgrade to down duvets ala the Four Seasons.
This would have to be a reform initiated by the management companies or linen services themselves. The owners cannot possibly have this level of control. Even if an owner has two sets of comforters or covers, it would require housekeeping to take the comforters off site every week, clean them, and put them somewhere where they don't get lost or mixed in with others. I do not run a management company but I would venture to guess that for most of them that would be a nightmare and I would bet they are unwilling to do it.
Until such a standard becomes commonplace, the best the owners can really do is see to it that the comforters or duvet covers are washed as frequently as possible. I have always believed that if you are renting your home, you should not have anything too precious, and everything should be easily washable if the guests so desire.
Once again, we DO want our guests to enjoy themselves and feel as comfortable as possible.