IOPS/TB is an important metric for hard drives these days. Capacity increases have been pushing that number down, making the performance characteristics of larger hard drives incrementally more tape-like and less suitable for live data. Splitting the actuators gets hard drives back to the IOPS/TB they had a few generations ago, which makes them suitable for a slightly broader range of applications than they otherwise would be. It also means that an older system using an array of 8TB drives can be consolidated onto half as many 16TB drives with minimal other architecture changes.
(Decreasing IOPS/TB is also why SATA SSDs haven't gone beyond 8TB while NVMe and SAS SSDs are pushing past 30TB.)
I'd want to see an overall calculation. Twice as many actuators means twice as many things can go wrong. I think I prefer KISS. On the flip side, I guess each actuator gets used half as often.
I use HDD for bulk storage and SSD for high-speed storage. I don't mind less speed, except insofar as it impacts reliability (e.g. a drive in a RAID fails, and the redundant drive fails while rebuilding the array).
It seems like competitors to HDD have become increasingly noncompetitive over time. Tape drives are insanely expensive at reasonable capacity. DVDs hold 0.1% of an HDD and are more expensive. Various newfangled optical drives (e.g. M DISC) appear more reliable than HDDs, but also cost much more per TB. Plus, you need to keep a big pile of coasters.
I don't think that will be true. By having the heads only read half the platters, the bytes per track are <areal density increase>/2. That's more seek operations for linear reads and small offsets. For completely random access that's probably the same number of seek operations as before, but queuing latency is lower.
Where it might reduce seeks is where multiple processes are trying to stream data at the same time, which sounds like is happening more and more.
Dual actuators don't actually double IOPS/TB for all sorts of reasons - queuing effects, contention for resources elsewhere in the drive, or limitations of the external interface. In practice it tends to be far short of double, so you're better off planning for larger numbers of (possibly smaller) drives with the same capacity and IOPS than for single drives getting faster by any measure.
(Decreasing IOPS/TB is also why SATA SSDs haven't gone beyond 8TB while NVMe and SAS SSDs are pushing past 30TB.)