I've seen it several times. Especially with cards that don't help your deck, like Sea Hag.
That is, to put it mildly, an unconvincing example. Sea Hag is very often skippable, in 2-player or multiplayer.
Re: Militia, agreed, that's not an attack that scales in multiplayer. Though it's safe to assume not everyone will skip it, since getting the first attack out there is still good, and people aren't going to let that first player be the only one not affected by a handsize attack.
Re: good junking attacks (Witch/Cultist/Torturer etc., rather than the weaker ones like Sea Hag or Marauder), the argument seems to be as follows. In 2-player, both players must get them. In multiplayer, it can be reasonable to ignore them in favor of some other useful card. Put in more concrete terms, you're saying that on a board with Witch and Lab, in 2-player your first $5 is always Witch, in multiplayer one should take Lab. Am I correctly understanding the argument being presented here?
In my experience (playing this game for ten years, almost exclusively multiplayer), that is not correct. Eating 2-3 curses/ruins per turn will cripple you, and the "but when the junk runs out" end state is irrelevant because the game is over. Remember, piles run
really fast in multiplayer.
If the card you buy instead of the junker is a trasher like Sentry or Junk Dealer, yeah, that could sometimes work. But if something like that is on the board there's a decent case to be made that they should be the first $5 you buy anyway.
Suggestion for those of you in the "attacks are weaker in multiplayer" camp: play the Invasions recommended set (Dark Ages + Intrigue). Compare 2-player and 4-player, particularly as player 4. That was the most memorable, though not the most enjoyable, Dominion game I've ever played.
At least in 2-player there's a chance shuffle luck can keep you afloat...