I said earlier: Probe in Modern is different than Probe in Vintage.
Dissect this a bit. Probe in Modern is played in Infect, Death's Shadow Zoo, etc. These are combat-based, weirdly interactive decks... that are nevertheless really not super-interesting or fun to play 100 times in a row. They have "lots" of lines of play but in the end they are just variations of the same theme - stick a creature that's a lot bigger than it should be and try to avoid combat.
Additionally, it's actually kind of perverse that Probe's -2 life is an advantage in Zoo and a total non-factor against Infect.
Probe in Vintage has some pretty crappy applications in the fringe DPS deck and the hopefully-soon-to-be-fringe Outcome decks... but that's a small slice of what it is. In the main, it has some pretty interesting implications in the grindier Gush decks in terms of fractionally modifying their curves, being more than one thing (not just with the tokens... but with applications like Cabal Therapy) and informing play sequencing. It's fun. And the life loss is starting to matter as things unfold with Delver and Eldrazi etc., so "opportunity cost doesn't matter" isn't necessarily true?