I know two companies running running openshift on Mesos. The reason modern Paas aren't enough, is the factat some scale you're not just running web services. But also rather complex data pipelines involving distributed data systems like kafka, spark, cassandra, etc .. and a simple paas has issues handling those workloads. That's where Mesos shines.
As for the user value, I consider having an uptime system, providing reliable and intelligent service based on data processing, by efficiently using your resources is has an important business value.
I find Mesos the most interesting because it's a kind of mirror image of Lattice[1], particularly Diego[2][2][3]. Both of them push intelligence out to the edges, but in different ways.
Diego pushes the scheduling problem out to the executors themselves through an auction mechanism. Mesos delegates it back to the requestors, as I understand it.
The way it was described to me by Onsi Fakhouri is that Diego is "demand-driven", and Mesos is "supply-driven". Diego grew from the lessons learnt on Cloud Foundry v2, so it favours fast placement of requests over perfect fit. But in the fact of the network fallacies, it probably turns out to be a good approach anyhow.
By way of warning, it has been a few months since I read the relevant papers and my memory is fuzzy.
Edit: and it looks as though the auction mechanism was moved away from. Hm.
