I've used BiPAPs in regular mode and auto. I think the benefits are much the same as with CPAP vs. APAP: if your pressure needs change based on position or some nights, it can be great because you can usually use lower pressure. Some people like auto better, or get better sleep on auto. Some sleep better on straight pressure.
With CPAP machines, the auto option is really not much of a difference in price, so it makes sense to get it. That may not be true with a bipap.
I'm not sure what could possibly be "dangerous" about an auto bipap. You have most of the same issues with auto CPAPs, and many, many people use those.
I would say, though, that I wouldn't get too involved in getting the "best" machine as an auto. Most of the time, a basic bipap will do just fine. The auto part mainly copes with pressure need changes through the night. If you have enough pressure set, you don't need more pressure, so no need for auto - for most people.
Personally, I had an auto bipap before my ASV, and the flexibility in trying stuff was very nice to have. I liked the auto mode, but I also watched my data very, very closely. If I wasn't a data hawk, I would probably use a set pressure. In my case, I had a lot of issues with xPAP therapy so the flexibility was important, but I can't imagine that being the case for most people. :)
I'm not sure why your DME is saying you have to have a prescription for "auto bipap". It's usually the same insurance code for any bipap. They can set it up for straight bipap, and maybe you can pay the difference in price. Then you could start with straight bipap, and have the option to try auto if you need.
Just one user's opinion. :)