I've used assorted serial port concentrators (digi included) to get remote access to devices in infrastructure I've deployed in the past, but not generally to something like a Service Processor that was already providing out of band management. The exception would be where there was a design requirement to have modem level access to an environment in the event of a total loss of network connectivity.
Unless you fall into that relatively extreme scenario, I would design redundancy into the switch paths so that your in-band access (e0a, etc) and your out-of-band access (SP/e0M) are via diffferent switches or network paths. Also look at things like connecting node-a SP and node-b SP to different switches. This protects you from assorted switch failure scenraios.
I've also on occasion run the serial consoles to nearby servers just in case. Often a nearby management server will have console ports available. Managed PDUs with port level power control can also get you out of some fault scenarios when the installation is just too remote, too innaccessible, or simply unmanned.
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