As you know Bitcoind can run in pruned mode, thus not advertising blocks to other nodes. Even though it has some drawbacks, saving you storage space isn't a benefit you can ignore.

My question is about the magic 550(MiB) default/minimum number. Why they've selected that limit? (& Why not less or more?)

I know it implicitly indicates blocks will be kept for the last (about) 3.8 days (91 hours) (one MiB because of one block every 10 minutes). But where that number come from? I know there is also a dust limit equal to 546 Satoshis.[182*3 ~ 550 here! Even though it is unlikely to be related to this case]

Would you please explaining what the logic behind 550 is? Is there any kind of difficulty level relating to 51% attack or any other stuff like that?

As you know Bitcoind can run in pruned mode, thus not advertising blocks to other nodes. Even though it has some drawbacks, saving you storage space isn't a benefit you can ignore.

My question is about the magic 550(MiB) default/minimum number. Why they've selected that limit? (& Why not less or more?)

I know it implicitly indicates blocks will be kept for the last (about) 3.8 days (91 hours) (one MiB because of one block every 10 minutes). But where that number come from? I know there is also a dust limit equal to 546 Satoshis.[182*3 ~ 550 here! Even though it is unlikely to be related to this case]

Would you please explaining what the logic behind 550 is? Is there any kind of difficulty level relating to 51% attack or any other stuff like that?

Edit: From 0.12.0, bitcoind supports relaying NEW blocks to other nodes, thus the first assumption up there, isn't an accurate sentence!