Let me jump in here on this one with a story.
I was diving in a local quarry a couple weekends ago. The water was 77 degrees, 20 foot visibility down to the thermocline, which was at 30 feet. There were two other guys diving there, one of whom I know to be an instructor at a LDS and the other guy who I didn't know.
They came up and were doing a short surface interval in the water.
The instructor said "Thats like, what, your 58th?" The other guy said "Yeah, that's right"... the instructor said "OK, then we do two more dives, go to twenty feet and stay down 20 minutes, so I can sign off on your book and we can get out of this crap. I hate diving in this crap"
Turns out they were out there doing a series of 20 minute dives to 20 feet just so the guy could get to the minimum required dives to be a DM.
I've got a LOT more dives than this DM candidate had in, going on twice as many, and not a single bleeding one of them have been of the "20 feet for 20 minutes so I can count a dive" variety... and I am not qualified (IMHO) to even START divemaster training.
Standards don't mean anything if people are willing to skirt the spirit of the rules, unless the agencies make people take rigorous individual skills tests with non-partial observers doing the testing.
People pay a lot of money to take these courses, and I think a lot of LDS's are really hesitant to fail anybody... which is why when I do eventually take my DM training I am going to go back to my original instructor who wouldn't even accept me unless he thought I was skilled enough to meet HIS standards... and would spend the time necessary to make sure I stone-cold was ready to do it and succeed.
This whole "start DM training with 20 dives" thing is a joke... the whole "number of dives" thing is a joke as well, since that is so easily (and often) manipulated.
A better standard, IMHO, is a skills review to start out with. I have been in the water with a BUNCH of DM candidates who simply don't know how to dive... and several certified DM's who frankly look worse than some new divers I have seen.
Personally, I am going to want to dive in about every condition before I feel I am read to be a DM... from drift diving (or float diving if you prefer), wreck diving, strong currents, surge, swell, cold water, zero vis, you name it... I think unless a person has a lot of EXPERIENCE in challenging conditions (and I don't yet), then they are not really ready to be a DM... but that's just my humble opinion.