you play against, say, a 'nid player, a space marine player and a tau player then tailoring yourselves to fight one-another will leave you, eventually, with a list that can deal with hordes, CC armies, shooty armies, varied armies, probably flyers and FMCs.
In my opinion, tailoring is bad sportsmanship IF the other player isn't fine with it.
I certainly agree that tailoring is bad sportsmanship if the other player doesn't know and have an opportunity to do the same back.
To your first point which I quoted, and to what Bits had been saying, I think there is a reasonable difference between rewriting your army to handle new situations or problems. That's part of learning and growing as a player and building an army.
Tailoring looks like bad form when it results in a list which is really useful against its intended victim but of less use against others. When I have an army which is exactly what I need to beat my friend's Eldar army, but would get stomped by most marine lists or ork lists then I might be guilty of tailoring.
If by contrast I analyze the tau, nids, and marines in my group and write a list that can handle all of them, then what I really have is the makings of an all-comers list. I haven't written a list especially for those tau at the expense of my ability to fight the bugs or marines.
Along this line of thinking we eventually get to a point of maximum utility for the army book. Where a list is about as effective as it can be at a given points range within the choices available. And then we're in competitive territory that
can speak more eloquently about than I.
There is a difference between writing the most competitive possible list and tailoring your army.
To the OP's point, revising your list to better cope with a difficult enemy is not necessarily tailoring.