Use Senior rates a lot, all over the world.

But I have not seen both scenarios you mentioned.
When you compare the senior rate you need to compare it to the flexible rate as they both are fully cancellable without penalty within the deadline allowed. I have yet seen any senior rate more expensive than the flex rate across 2 doz or more properties booked with senior rates the past 9 months.

1) never see Senior rrate at 15% off the regular rate, but almost always see senior rate slightly better than the flexible rate, and often the same as non-refundable rate, but is refundable. This is valuable to me. I rarely book non refundable rates so comparing to non refundable rate is meaningless to me, and probably to most people who use senior rate.

2) the time I see Senior rate is significantly higher than "regular rate" is when the "regular rate" is nothing regular, but a "flash sale" of nonrefundable rate. I see one at this moment - 4P at Perth in the first week of October - the senior, fully refundable rate is 150 AUD but a "flash sale" under the "package and deals" tab is only 110 AUD. That is a significant difference.

If you can take the risk of non-refundable booking, then this should be a good deal, PROVIDED the "flash sale" also earns stay and point credit. I dont know about the latter.