My TAC experience is admittedly out of date, but out of hundreds of clusters I worked on, maybe 5% were any kind of load-sharing (including StoneBeat, Nokia IP Clustering, ClusterXL unicast, and ClusterXL multicast).
I'm honestly not sure how many companies host their own email anymore, but I haven't personally used an on-prem mail server in close to a decade. My company's current mail filtering is all externally hosted and mail traffic doesn't touch my firewalls.
My managements are still R81.20 rather than R82, but that's the only reason I couldn't use the 3900s. If ElasticXL is added, we'd definitely be interested in some for offices.