Players use similar strategies in singleplayer, for the same reason. Because AI France has a bigger manpower pool to start with than any of its neighbors, and due to its bonuses takes fewer casualties than its neighbors in wars, it can declare wars every five years at near-full manpower while its neighbors have just finished reinforcing their stacks and are only after five years building up manpower buffers. This is one of the attack patterns I saw regularly used by AI France. The truce changes seem to be fairly solid, because it allows countries more time to recover from disastrous wars, so that the first war is no longer nearly-guaranteed to be decisive. But the game should not be gimped solely because the AI is not as good at it as the player. EU4 AI is quite good already, and it has been improving steadily with each patch. If the AI cannot properly play the game, then that is a task for the AI programmer. In short, you balance and test the game in multiplayer because players put much greater stress on the system than any AI. In contrast, Civ5 multiplayer was barely functional on release, and the game was clearly designed almost solely around a singleplayer experience and Civ5 is still a much weaker game compared to Civ4 in terms of balance and mechanics because balance issues that would have been caught by proper stress-testing were concealed by an incompetent AI. Some parts of the game, such as the entire Civ4 scoring system, were designed almost wholly by multiplayer testers. For example, Civ4 was exhaustively tested in multiplayer, revealing many design problems that were subsequently resolved. If properly done, multiplayer testing will lead to a much stronger game than testing around an AI and its inherent limitations.
MP is one of the best ways to balance a game because good players will seek out and exploit every potential weakness in the design. Sometimes, I'll see someone who is arguing on the side of a mechanic I believe in, but the argument is just awful or the logic is flawed and that sucks because I want to call them out on it regardless.
By far the most frustrating thing is when post after post is just another yell of "make it this way or that way". I've been beaten in debates here quite a few times (the phrase "written daggers" comes from experience -> it's uneasy being put in a position you can't argue out of, and people have done it to me here), and that's part of what keeps me coming back along with the game's actual strategy discussion. For every well-reasoned point brought up by either side, you have what, a dozen statements made without basis or are pure fallacy or insults? I admit I'm no angel in this regard, because my frustration over that gets the better of me more than I'd like, but I'd honestly prefer to drill discussion and not insults. It's one of the biggest frustrations for me here.