Yes, that’s the status quo actually. And to be honest the “standardization” of media file metadata never was in a very good shape. It is often more driven by usage of big players, namely iTunes and Windows Media Player. Developers of other soft- and hardware often cared more about compatibility with those then any existing spec (which were never formally standardized anyway).
If you have to decide for your tagging software what tag / frame / atom name to use for some information you should really look at how other software is doing it, because there is no use of writing information nobody else can read. There is a reason why the ID3 developer site lists Unofficial Frames Seen in the Wild (including TSOP, TCMP, TSO2, which are all used by Picard). There are multiple examples of deviation from the specs, e.g. the use of TPE2 (Band/orchestra/accompaniment) as Album Artist. This is not following the words of the spec, but it is how virtually all music players and taggers treat it if they support the concept of Album Artist (and it was probably started by one of the usual suspects I listed above). Also rating via POPM never really got used as intended by its inventors, which wouls allow multiple ratings for different people in the same file.
Update: And since you mentioned just using ID3 v2.4 instead of v2.3: The very same software that is responsible for some of the current tag usage used to be notoriously bad at supporting ID3 v2.4, so this was not an option for many users. Overall v2.3 is still the more compatible, although I would recommend using v2.4 unless you have some component that doesn’t support it properly.
Yes, the same tagging is used for Quicktime files, too. To my knowledge the underlying metadata structure is technical identical. In 2008 Apple published a document called “iTunes Metadata Format Specification” (it used to be available via their developer site after registration, unfortunately the last time I looked for it it seemed to be no longer available) that documented the use of metadata atoms for MP4 files. Picards usage of tags is in large parts based on this.
At the moment if you use Picard to tag Quicktime files they will use exactly the same tags. I think this is currently what most software does (if it supports older quicktime files at all). There might have been other tag names in use before Apple formalized the tag names a bit more with the metadata spec. But I never heard of anyone requesting something special only for Quicktime.