Actually I have changed my mind
All the RDF can tell you is what bearing the beacon is from your aircraft. It cannot tell you what radial you are on. Sure if you set out from the beacon heading 150 by your compass then you will be on the 150 radial and the RDF will show the beacon dead astern at 180 (relative to your aircraft). The problem is that if you stray off course then pointing your tail back at the beacon will just make your error worse. The RDF will show the beacon is 180 relative to your aircraft but you are now heading 160. You are on the 160 radial now but without delving into maths you don't know how far along it. The only way you can find the target now is to use the compass and landmarks. If your deviation was actually caused by a crosswind then your required heading will not be along any of the radials!
Before anyone leaps in, Yes a good navigator would be able to use the relative bearing of the beacon in conjunction with the compass heading to work out how to get back on course but it involves plotting and maths which is what this method was supposed to be avoiding.