Is the concern about wooden racks is bonding? If a metal rack is attached to a wooden rafter how is that different from a metal rack connected to wooden cross piece on ground mount?
If something is not conductive, then you do not have to use bonding and grounding. There are some systems that have no exposed metal parts, so they do not require equipment grounding. I have heard rumors that more of these systems are being developed.
If you are using a wooden racking system and PV with aluminum frames, you will still have to make sure that all of the aluminum frames are bonded.
A drawback to wooden racking systems, is that you are much more likely to be required to have an engineer stamp your plans. With the mass produced metal rails, there is already engineering that goes with the racks. Also, like Marco pointed out, the wooden racks can burn.
Usually when you see a wooden rack, it is an old PV system. The 20 year old system that I show in this course on the far end of the property I live in is a good example of what can happen to wooden rack after years.