TLER is relevant even on "software RAID", because fileservers generally should not go catatonic. TLER is simply more important on hardware RAID because failure to have it may cause a controller to decide the drive's failed.
One interesting thing to note is that a fair number of "prosumer" or "power user" NAS boxes are entirely software driven, and WD has aimed the Red series at these guys as well as the slightly upscale units that might have some minimal RAID hardware. For example, units like the Synology DS1512+ have the Intel ICH10R, but for error recovery purposes that is still essentially a software driver based unit, the drives aren't going to go offline unless the system decides to take them offline.
I've been watching the storage market for many years, and the thinking has evolved that "RAID" implies "high performance" and "open corporate wallet". As such, features like TLER were often restricted to ridiculously expensive, fast, power-hungry drives. The irony here is that for truly massive storage projects, you don't need fast drives, because when you've got 48 or 72 drives in a box, even with 10GE, there are limits as to how much data you can shovel around, and when you're setting up many of these boxes to provide Internet services, your bottleneck simply isn't likely to be the individual storage devices. You also don't want power-hungry drives. That has left designers of such projects with unpleasant choices to make. For example, the WD RE4 2TB model is around $250, while the Green 2TB is around $100. That can make a huge budgetary difference when you're buying two cases of drives for a single server.
Yet, and here's the interesting point, when you have even just 24 2TB drives, are you storing small files or large files? Because if you're storing small files, the difference between 5400RPM and 7200RPM may matter, but for large files, basically it doesn't. If you start running 50 megabits of data into your 40TB array, that's 6MB/sec, and it will take 77 *days* to fill. Touching each location on the disks only once (in general). On the other hand, if you're storing small files, then seek latency comes into play, and you'll discover that you actually cannot fill the disks before they are statistically likely to fail.
So, from a large scale storage system designer's perspective, the "RAID edition" drive strategy from manufacturers kind of sucks. For some time now, the pragmatic design choice has been to buy "desktop" or "green" drives, because that was where 5400RPM and reasonable prices and better power budgeting was. And now it seems that WD has stuck what is essentially RAID firmware on "Green" series drives, at a modest price premium, which is an attractive option for some of us where no good options existed before.
Really, what it comes down to is that I think they finally figured out that desktop computers are slowly dying, the new generation of NAS devices have little need for super-expensive hard drives, since they're almost universally unable to exploit the full potential of a drive anyways, and there's some money to be made by offering a 5400RPM "RAID" firmware drive.
