When I was doing vendor vetting the talk usually revolved around the "blockchain revolution" but glossed over over the consensus issues. In the permissioned blockchain space, mining is really not an option (how do the incentives for mining work when there are no coins involved?). Luckily, we have a lot more flexibility in the consensus system we can pick.
Moreover, the term "blockchain" itself is ill-defined/overloaded. If all blockchains use mining then the 51% issue is ever present (though if I remember correctly problems arise at 30-ish%). If the broader "BFT consensus" definition is used then no, you can use PBFT/SmartBFT and a host of others to come to consensus without ever needing to touch crypto let alone mining. The issue, of course, is that Mining-based consensus is the only system that works for truly public blockchains, as it allows for anonymous participation at all levels and can also scale flat.
If anyone else is a consensus nerd, I wrote a technical-biz-persone level white paper on permissioned blockchains that goes into more depth and covers the issues associated with using TLS vs PPK sigs [1].
[0]: http://github.com/kadena-io/juno
[1]: http://kadena.io/docs/Kadena-ConsensusWhitePaper-Aug2016.pdf