If you like you can try it out on DGS (though not real-time) with shape-games to live in the center:
http://www.dragongoserver.net/view_shape.php?shape=10
http://www.dragongoserver.net/view_shape.php?shape=16

Agreed. I think beginners are often told it can't be killed because in the examples given, trying to kill results in a worse result, but it is possible to hold the invasion to one eye in exchange for the invader breaking the seal. Whether you can prevent them from getting a second eye afterwards in the center depends entirely on how strong you are there. Killing this can also be rather complicated, so it's much easier not to mention it until later, as the circumstances are relatively rare and up to a certain level, one is liable to let it live and end up with a much worse position than otherwise.

And here's an example of Takemiya killing the 33 point invasion: http://senseis.xmp.net/?TakemiyaMasakiK ... ntInvasion

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Occupy Babel!

Why am I not surprised that Takemiya would be the kind of pro who would know when he can kill a 3-3 invasion?

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"You have to walk before you can run. Black 1 was a walking move.
I blushed inwardly to recall the ignorant thoughts that had gone through
my mind before, when I had not realized the true worth of Black 1."
-Kageyama Toshiro on proper moves

In that example, I think both players knew it could be killed, but that Okada used it as a sacrifice to enable a greater reduction from the outside than he would have been able to accomplish otherwise.

Moderately strong amateurs (5d+ on tygem and oro) know when you can kill a 3-3 invasion too.