You can do that already if and when you prefer. And if you don't like _ALL or _TAG at all don't use them. For me they can be extremely useful in some cases.
How much you need _ALL (or _TAG) with e.g. "Split field by separator" depends on the use case. A little background:
I was inspired by this recent thread where the $list function was mentioned (by ohrenkino actually 
). I started to build two action groups for export and subsequent import of tags. But since the $list function separates multiple field values with , (comma + space), problems arise when importing the tags back - i.e. when the values themselves also has commas, it's impossible to import them back as multiples correctly without a workaround. The solution was to merge all multiples with a special character before using the $list function, and afterwards split them back by the same character.
The merging with the action "Merge duplicate fields" could be done by using _ALL. But the action "Split field by separator" cannot utilise _ALL from the listing, hence my bug report here. Bug or not, it would help a lot if _ALL or _TAG could be used so you wouldn't need to create separate actions for splitting each and every field, when so many of them are considered for splitting.
By the way, pitfalls are easily avoided in this case by using a very unique character as a separator. One shouldn't use the common separators you mentioned of course.
Come to think of it, an alternative solution would be if one could specify the multiples separator for the $list function. The comma isn't ideal as a separator if there are commas in the values themselves.
Anyway, my point is that "Split field by separator" has many use cases, some of which would benefit a lot from _ALL or _TAG.