MAME's set policy is

rompath\setname\rom1 ... rom n
samplepath\setname\sample1 ... sample n
rompath\setname\chd11 ... chd n

and

rompath\setname.zip
samplepath\setname.zip
rompath\setname.zip (chds in zips...not useful...)

That's what it has been. The 'new' way: rompath\chdfile was accidently added when chd support for clones was added. Since I think it totally breaks mame's set policy and breaks up the relationship from set and its belonging chds (e.g. look at the beatmania chd names...you can't see any relationship if they're kept in a rompath without subfolders), I won't support it.

In fact I asked some devs about it and they even didn't know about it. They were surprised as well.
Maybe it will be removed, maybe not...but currently I don't see a reason to support it. It's not logical.

Roman Scherzer
ClrMamePro