I am trying to put this into perspective. I realize I am talking to a group that for a large part gives little respect to professional technicians. Part of that lack of respect starts with ignorance. it is hard to understand how complicated these systems are when you have little knowledge and no experience.
This one subject is fairly matter of fact, so it is possible for me to place a perspective on the view each gets when using an OBDII point of view. I can tell you that I almost never talk to a MB in OBDII. I can also tell you that about 98% of what I observe when using a diagnostic communications device (scanner) can not be observed using OBDII. Ninety percent of that 98 comes from the fact that I'm working on things other than engine performance most of the time. The other 8% of the 10% involved with the engine is not available with the generic language of OBDII.
The modern car is a network of systems, having a foggy access to one module is almost dangerous from a diagnostic standpoint. But better than being totally blind I suppose. I would not buy parts based on an OBDII read out, but I might decide if I could go on a trip based on a OBDII evaluation of a check engine light.
I do own one of these:
http://www.crecorder.com/website/index.do?method=welcome
It gathers about 20 OBDII data points continuously for a huge amount of time. One can then graph the results using their software. I don't do it often because as I stated only a few items are the same in OBDII as in MB. Watching fuel adaptation in OBDII is frustrating as one or two whole components are not represented.