I can't speak for some of these (like craigslist-- until recently I have lived in very out of the way places). But for others, I have existing solutions that work well. For example, in the case of being owed money I just let them pay me back whenever. Generally it works out that maybe we go to play poker and he pays my buy-in in exchange for that lunch I bought him, or he grabs me a drink when he goes outside for a smoke, or some other timely, convenient thing.
Such is the case, I guess, for pretty much everything. Bump itself replaces things that already exist, but in its case it seems like it can do clearly better with few barriers to adoption (e.g. games), while with the paypal scheme it's not so clear. My reactionary tendencies pick up, "don't fix it if it ain't broke". But, you're right, time will tell. Just because the existing solutions seem perfectly fine doesn't mean Paypal's system won't become easy enough and commonplace enough for everyday use. Because, yes, it does have some advantages.
It still seems so pointless to me, but then I guess, so do/did a lot of established things.