Lots of people are using Tendermint. The biggest users would be Hyperledger Burrow (https://github.com/hyperledger/burrow) and our own Ethermint (https://github.com/tendermint/ethermint). We're also working on an SDK for building apps in Golang (github.com/cosmos/cosmos-sdk) but it won't be ready for another month or two. There are a variety of apps built by members of the community, and ABCI servers in different languages - you can see a somewhat up-to-date list here: https://tendermint.com/ecosystem
Private blockchains are incentivized to adopt public blockchain architectures because public blockchains are most economically vetted.
Regarding private blockchains growing to public trustless blockchains, that is not a trivial problem. There are new blockchain models that are being promoted (e.g. IOTA, EOS, NEM) which have not being studied enough by consensus researchers.
I don't know of anyone who really applied traditional BFT networks into public blockchain architecture, however, until Tendermint.
For example, who spoke or wrote of BFT fault accountability/attribution, and how it pertains to proof-of-stake models, before Tendermint?
There was also the 2011 paper that lamented the lack of good open-source middleware for BFT algos. Tendermint solves that engineering problem as well.
Looks pretty product-focused to me.
Using gRPC for Block Finance (http://block.finance).
It would be interesting to hear from people who actually use (if there are any) Tendermint: * How easy/hard is it to create bindings to the blockchain from your own backend application (as I can see there is a GRPC option). * How many nodes you are running? * What's the use case?