I can't tell if you think that's a good idea or a bad idea.
I run a short every four hours and a long twice a week. The SMART tests may actually be better to run as the HDD manages these and is supposed to keep them from interfering with the disk I/O demands of the system, and does the entire disk linearly. Scrubs do not analyze the unused portions of the pool. I would rather know sooner that a problem is developing.
The full smart test is still going to strain a disk, when I'm asking to read or write and a disk has been in the middle of reading all 5TB for 5 hours, the heads need not continue to thrash around performing the test. Ideally the disk would go offline entirely.
I'm happy with monthly SMART tests, it's VASTLY MORE than I ever gave disks before I had a NAS (no smart checks ever, then wonder why a disk fails 'suddenly' losing data)
So yes a good idea, once a month seems reasonable enough.
Scrubs may be a bit tougher on the drives because there tends to be more seeking. Hard drives should be able to do massive amounts of read activity without incurring a significant shortening of life expectancy. Some people have pointed to the recent trend to define data center HDD's in terms of workload TB written per day. This is mostly a side effect of SMR impacts and is for writes, not reads.
Honestly I agree with you, seems reads, really shouldn't impact the disk too much, but at the end of the day, the actuator still needs to work, the head is performing a function, it's utilisation more than idle.
Either way, since moving to FreeNAS, I've managed to have a reliable store of data and confidence in my hard drives.
In the 'old days' before FreeNAS I would go and buy a nice big 2TB hard disk and fill it 65% of the way, not knowing that 80% onwards was utterly worthless, damaged sectors, debris, just poorly formed magnetic surface, who knows - the 'end' of the disk was faulty on more than one occassion for me, but I NEVER encountered it until the last minute.
Now, I'm 99.9% confident my disks are reliable and I'm told of a problem with the disk itself,
BEFORE I've ever tried to write to that sector (!!)
Love it, very happy with FreeNAS as a storage OS