Yeah, if you just want a full history of all your transactions, and don't need the ability to spend any funds and/or sign any transactions... importing addresses into a watching-only wallet would give you that info.
Full history and sign transactions is the only purpose for this wallet further on. I don't know if I ever need but I think it's good to at least have the possibility.Signing messages with P2SH addresses is "a bit of a mess"... The primary reason being that there is no guarantee that a P2SH address actually has a private key to sign the message with... as they are a "Script Hash"... and the script doesn't necessarily involve private/public keys. Some wallets like Electrum, Mycelium and Trezor support signing/verifying using P2SH addresses... but it can be very inconsistent across implementations as there is no real "standard" 
Indeed, if you attempt to sign a message within Armory using one of the P2SH-P2PK addresses, it will give you a warning that this is not supported:
https://i.imgur.com/50k32Hk.png
So, "Proving ownership" is going to be difficult... I would suspect that if you needed to do so, you would have to deposit some funds into that address and then send a transaction from the address to prove that you have control over it.
Indeed, if you attempt to sign a message within Armory using one of the P2SH-P2PK addresses, it will give you a warning that this is not supported:
https://i.imgur.com/50k32Hk.png
So, "Proving ownership" is going to be difficult... I would suspect that if you needed to do so, you would have to deposit some funds into that address and then send a transaction from the address to prove that you have control over it.
That's a good option! Only downside of this is that I can't get away from armory (a full node with a growing blockchain). Because the problem with importing some of the private keys