True, but a queue depth of 1 means serial operation by definition, i would argue. Its also true many software use blocking I/O and parallel I/O is not always possible generally due to lack of advanced programming skills or lack of interest in the area. Games could be storing information in such a way so that they can read sequentially instead of a random-like pattern. And they could be using a different design to increase the amount of queued I/O's so RAIDs can take advantage of this.
But ofcourse, as a customer considering RAID you look at what performance you can get and not what is theoretically possible. If you use Windows all you can do is fix the stripe misalignment and hope applications allow the RAID to work in parallel. If that is not possible, then an SSD might be a better option because of its low latency any serial operation is going to be much faster than any RAID ever can.
One strange thing is that in my benchmarks done on BSD, i did get a higher IOps rating when using RAID0 and testing with a queue depth of 1. I have no good explanation for that. Ofcourse values increased as the queue depth increased, but with 4 disks in RAID0 i got about 70% additional performance with a queue depth of just 1. Tested this with software RAID.
I was thinking, perhaps a virtual I/O (done by an application) can be split in multiple physical I/O's going to the storage volume, which is something the Windows APIs do and also the ATA driver. I also know FreeBSD to do this; if you write 1 megabyte to a raw device the ATA driver will write chunks of 64KiB. I can also see the queue'd I/O's rise when benchmarking this way (using dd if=/dev/zero of=/dev/sdX bs=1m). So it seems a queue depth higher than 1 can be caused by the filesystem and/or operating system, not just the application.