Just a personal preference, I already have Deponia and Deponia Doomsday as separate games, so, for the sake of consistency in the library, I'd rather just buy the two games I am missing : Chaos Deponia and Goodbye Deponia, they often go on sale and don't cost a lot either.

And besides, I'd like to be able to track my playing stats for each game, and there's little point in a compilation when they've released a fourth installment that is not included there.