Agreed. I actually use some exercises that are geared specifically toward coordinating both hands and foot (feet), that start at a simpler, more manageable place than learning paradiddles while playing quarters. Like, singles at various note rates, alternating between one hand and one foot, and so on. It works well.From the standpoint of day one, trying to work hands and feet at the same time, I would agree that it isn’t useful. If day one, someone was practicing one foot at a time to build up the leg and core muscles to properly build toward playing heel-up both feet at the same time with proper posture, I think practicing slow basic beats is very useful. It can take months to work the feet out, independent of hands. So, starting early makes sense to me.
Although the paradiddle sticking pattern (RLRRLRLL) is easy to learn, the technique is pretty complex, because each hand needs to execute a downstroke, two taps, and an upstroke. This combination requires considerable control over the wrists, fingers, and fulcrum. I usually have the student practicing each hand by itself, and then also removing some notes as a "builder" exercise, to really shine a light on the stick movements, and to build muscle memory. I wouldn't expect that student to do all of this work while also playing quarters on the bass drum. That can come a bit later.