Quote from: zylstra on February 02, 2018, 07:07:12 AM

My understanding is that Segwit transactions that are included in legacy BTC blocks do not contain witness data.

When segwit transactions are related to legacy nodes, the witness data is stripped so they can be valid under the consensus rules that legacy nodes follow(i.e 1 million bytes block size limit) That was how Segwit was deployed as a soft fork.

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Why then include witness data in Segwit transactions that are included in Segwit blocks?

The so-called "witness data" is essentially the scriptSig(signature, pubkey, etc)
If the witness data isn't included in segwit transactions then how do you validate a transaction without the signature? Huh

The witness data can be over 1 million bytes, that's how the block size increase segwit brings comes into play.

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It seems like we get 2x transaction throughput for 4x block size, or maybe 2x block size on average. So why even bother with Segwit?

There is no block size anymore with Segwit deployment, but block weight.
We get 2x the "block size" as you put it, because that that was the requirement