I agree, and it is not just the PA28 (really needs multiple PA28 profiles too). The key functionality I miss at Simbrief is the ability to create your own aircraft profile and an easier way of sharing aircraft profiles.
That is one of the main reasons I still use PFPX for all my IFR flight planning, I can just create whichever profiles I need. It is more powerful than Simbrief in other ways as well, but that is the functionality I would miss the most. I used Simbrief for a while but went back to PFPX rather quickly.
The key reason to use a flight planner for IFR flights (even in a C172) for me is the alternate flight planning, especially having multiple alternates pre-planned or quickly change alternate just before flight if weather has changed. Second reason is if using airways and there are a dozen waypoints even for a short flight. Those things are rather tedious to plan with paper navlogs and my E6B. Still do it manually for VFR flight planning though.