As always, with any posture, you can begin by first asking what the intention of the pose is. Why are you doing it? What is the main targeted area? In Sphinx, the main targeted area is the lower back. In the modified Sphinx/Half Frog, are you still feeling some stress in the primary targeted area? If so, great! (I do, by the way.) But, with the Half Frog version, you may get some stress in a secondary targeted area: the hip. If you can get appropriate levels of stress in both targeted areas, then this is a good variation. Ditto for the Sphinx side bend or twist. For me that doesn't work as well, but for you it might. Another combo pose I like is the Banana is the Saddle: do a regular Saddle but add a side bend. (This only works if you can come all the way to the floor in Saddle, otherwise, don't bother.)
So - are these worth while? It depends? What do you intend, and what do you feel?