Note that all of Serhiy's examples are for a known, fixed number of things to concatenate/union/merge. str.join's API can be used for that by wrapping the arguments in an anonymous tuple/list, but it's more naturally for a variable number of things, and the unpacking generalizations haven't reached the point where:

    [*seq for seq in allsequences]

is allowed.

    list(itertools.chain.from_iterable(allsequences))

handles that just fine, but I could definitely see it being convenient to be able to do:

    [].join(allsequences)

That said, a big reason str provides .join is because it's not uncommon to want to join strings with a repeated separator, e.g.:

    # For not-really-csv-but-people-do-it-anyway
    ','.join(row_strings)

    # Separate words with spaces
    ' '.join(words)

    # Separate lines with newlines
    '\n'.join(lines)

I'm not seeing even one motivating use case for list.join/tuple.join that would actually join on a non-empty list or tuple ([None, 'STOP', None] being rather contrived). If that's not needed, it might make more sense to do this with an alternate constructor (a classmethod), e.g.:

    list.concat(allsequences)

which would avoid the cost of creating an otherwise unused empty list (the empty tuple is a singleton, so no cost is avoided there). It would also work equally well with both tuple and list (where making list.extend take varargs wouldn't help tuple, though it's a perfectly worthy idea on its own).

Personally, I don't find using itertools.chain (or its from_iterable alternate constructor) all that problematic (though I almost always import it with from itertools import chain to reduce the verbosity, especially when using chain.from_iterable). I think promoting itertools more is a good idea; right now, the notes on concatenation for sequence types mention str.join, bytes.join, and replacing tuple concatenation with a list that you call extend on, but doesn't mention itertools.chain at all, which seems like a failure to make the best solution the discoverable/obvious solution.