Hi Andy - It's not critical that you learn to plant. Not everyone does it (it's even a little controversial in classical guitar circles), and you can absolutely play arpeggios without planting. Having said this, there are some things that feel really uncomfortable and, as you put it, go against what our hands want to do, but which are nevertheless good for us to work on. In some cases, learning how to do something that at first feels so wrong is a path to growth. In other cases we're fighting a losing battle and are better off just moving on and ignoring it. I can't tell you for sure which this is for you, so I'll say that it's not a huge deal if you decide not to plant at all, but there's also a chance that there's something to be learned in the struggle, as for many players planting is very helpful :)
Cheers,
Kai