Europeans don't really run suicide Karthus nor do they see it as a strategy from what I've seen. They treat Karthus' passive like Anivia's; it's a second chance in case they make a mistake, not an excuse to go super aggro.
Also pro players dont sit in Karthus ult when he goes in suicide. They exhuast him and kill him and then move back and fight 4on5.
I generally agree with the assessment that Karthus' passive shouldn't be used to suicide willy nilly (it's pretty bad unless you can force a fight, ie. Jarvan ult), but in siege positions you should be suiciding to create short positional advantages if there's no major objectives out on the map. Doubly so if you have no way to force a fight.
Maybe it's just because I've had a lot of dominion experience, which contains constant situations with a defender's advantage, but if Karth is ever forced to lane swap out of bot, he basically has to suicide on points to ever contribute to a point attack unless your team has another good way to initiate (and even then, he generally converges to trying to die on point anyways).
Actually, it's even more generic than THAT. In Starcraft 2, there's a well known saying: "when you are ahead, get more ahead." In short, if you have an advantage, you should not take a risk to attempt to end the game immediately and instead leverage your advantage to turn it into an overwhelming one.
However, in LoL (well, in SC as well, it's just a lot harder to measure), there's an eventual limit to being ahead. If you have baron and the buffs, and you've filled all your item slots, that's it. That's as far ahead as you can get. So any progress on your opponent's side (and due to how it works, LoL has constant progress) at that point makes you less ahead. So you must take action before that point. You can't do what M5 did and try to leverage your advantage for half an hour.
M5 must have realised their only way get a tactical advantage was to poke them down with Corki (which wasn't working because they had too much heal and creep clearing) or to leverage the fact that CLG can't go where defile is, so the late game will heavily favour CLG if they're on the defence unless Karth suicides.
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