Looking at the PDP-7 maintenance manual, pages 3-19 and 3-20, the one's complement add (
ADD) and the two's complement add (
TAD) are implemented by a parallel half-add step (described as "xor") followed by a serial carry-propagation step.
The only difference is in the second step, where you need to preserve the sign and do a carry-around for one's complement add, while there's no carry-around, but propagation into the link for the two's complement add.
This means the additional one's complement add is comparatively cheap: all you have to do is to add some additional carry propagation lines, and enable or disable some of those lines depending on which version of the add you do.
All the remaining complexity of the PDP-1, that is, adjust of -0 to +0 in
ISP, is gone. (To be fair, most of those instructions are gone, too).
With this, we can speculate about the "why" (because unless someone can interview someone at DEC who was involved in this decision, we'll probably never know for sure): One major concern on the business side for every change is backwards compatibility. "But we do have lots of PDP-1 programs that use one's complement representation, will we be able to do that on the PDP-7?" "Don't worry, the PDP-7 has a one's complement add operation".
Of course, that ignores the fine detail that all the other concerns with one's complement like the two zero representations have now to be dealt with manually using more complicated skips, and requiring major effort when porting the programs. (But the business side, who makes the decision to buy a PDP-7, doesn't understand those details.)
So there you have it: A relatively cheap extension that can be a selling point to customers who are concerned about backward compatibility. (But as I wrote, all we can do is speculate).
To see some examples of real-world code, the PDP-7 Handbook recommends the following code snippets:
Two's complement of memory location Y:
One's complement subtraction A - B:
Two's complement subtraction A - B (using constant):
Note that the two's complement subtraction is more complex than the one's complement one (as the operate instruction doesn't include an increment, as e.g. on the PDP-8, though with the carry-propagation circuitry in place, it wouldn't have been hard to do).
I have not found a recommended "compare to -0 or +0", but I think you could do it (destructively) by shifting out the sign:
Given that skips are executed before the rest of the operate instruction, I don't think it can be done in one instruction.