What is the benefit and differences offered by Obelisk servers?
the libbitcoin framework is definitely meant for back-end development. If you don't want to compile the source from bitcoin-server, you can download the binaries for Windows, Linux and the Mac. See this slide presentation for an overview about libbitcoin.
Obelisk, now called bitcoin-server (bs), is build upon libbitcoin framework that is built on top of the zeromq and sodium packages. This means connections between bitcoin-server and clients such as bitcoin-explorer (bx) can be made scalable and secure the confidentiality of data in transit between bx clients and bitcoin-servers. The communication ports between bx and bs are different from regular bitcoin clients and their nodes.
However, bs still uses ports 8333 (mainnet) and 18333 (testnet) to communicate to the rest of the Bitcoin network nodes. When runnings bs, you can see easily see how the height of the local copy of the Bitcoin Blockchain is via watching the stdout. Transactions also start to appear on the stdout channel when the bs node is almost complete.
The Spesmilo repository for Obelisk is now deprecated by bitcoin-server. Similarly, the sx repository is also deprecated by bx. The latest code for bs and bx have very fast moving branches. Beware that bitcoin-servers tend to require a little over 3x the disk space than bitcoin-core servers for retaining the blockchain. I suspect an 8x increase in the max Bitcoin block size will require the blockchain format used by bs for storage to undergo a redesign.
"Obelisk is a scalable blockchain query infrastructure. A pure Python library is great because it can be deployed easily without requiring dependencies. This allows code to be ported across many platforms such as the mobile phone with Kivy, desktops or any platform running Python."
So it could be conveniently implemented in development of a processing service/backend.