You can indeed put non-volatile write cache into an array (well, technically speaking, battery backed up cache and, if the manufacturer is wise, either a buffer-write out to disk or full destage under batter power for long term power outages - as they found to their costs in Canada a few years back when mains power was lost for many days after an ice storm).
However, the array based approach is hardly cost effective for individual PCs. Even in the case of arrays, then there is a good reason for going this way. If each device has its own NV write cache allowing for full buffer destage (either powered from an internal battery/capacitor, or by a battery in the array), then a hhuge amount of cost could be removed from the array. All the arrays I know are configured with RAID protection. In that case, even if one SSD fails, then the data is still available. It then means that all that expensive mirrored and battery backed up write cache in the array can be junked as the low latency of writes to these new devices means that these can all be synchronous. There might still be a need for some of this mirrored memory - the array will have all sorts of state information to maintain, and you wouldn't want a storage system crash in the event of the failure of a critical component. However, even then it might be possible to do without it. SUN's unified storage appliance does not have an NV mirrored write cache. It has the option for write-optimised SSD for this purpose (in a resilient configuration), although node failure results in a disruptive failover as state information is not mirrored across and ZFS is not a clustered file system. However, that could be dealt with in the future.
If open storage platforms are to take off, then expensive, proprietary hardware like mirrored NR RAM probably need to go too.