Rope Wrench/RRP in SRT

I've really only done old school MRT tree access where I'm in a moving rope that's tied to me with a Clove hitch and back to the standing end with a Blake's hitch. I'm definitely interested in trying out SRT to take advantage of ascenders to walk up vs hip kick slide to the top but I'm not sure I want to spend $3/400 on a rope runner pro etc. I think I understand how they work, by absorbing ½ your weight so your hitch doesn't lock up on your way down, right? So if that's their purpose/advantage why can't I use a munter hitch on a biner instead? I already have lots of those.