I think in most cases, people either collect full panes, Plate Blocks, Blocks of 4, or single stamps. I'm not aware of anyone collecting half panes - but this is stamp collecting and your collection is defined by your preferences and interests.
For US stamps, who is selling half panes sealed? These would have to be removed from packaging if purchased from SFS and then half pane removed. I wonder how many collect corner blocks. Years ago when sheets had 50 stamps, margin blocks were collected especially with multiple plate numbers.
With the commemorative mini-sheets now, it would be very inconsistent in the multi-design sheets. Most dealers either sell a single or full sheet. The plate number is not that relevant as it used to be.
I actually remember buying a couple of these from SFS in KC … commemoratives for which I didn't want to buy a full pane. At the time you could often change the last digits of the item number for a full pane from 04 to 01 or 02 in order to buy less than a full pane … one of these was for a block of 4, the other for a block of 10. I don't recall which number was for which block size. Panes of 16 or fewer stamps were not made available in block format.
I may be incorrect here but USPS makes a differentiation between PANE & SHEET on the current commemorative issues. They allow a special order of singles, blocks, xx# of most issues (including commemoratives) produced in SHEETS. They do not allow the breaking up of PANES (or coils) in Special Orders for less than the original unit produced. They will charge a Service Fee for the extra effort. I also believe that any issue valued over $1.00 can be purchased as a single w/o a service fee (but that might Have changed). When I order from the Fulfillment Center I purchase the block of 4 most times rather than the full sheet. (I'm stuck with the full pane if that is the issue item)