Loose Crank Arm Problem

I've been having a very similar problem on a Sugino RD2 crankset. Tapers appear fine. Only using an Allen key to tighten, which may be the problem. Can anyone who knows about such things say why it's a bad idea to use threadlocking compound (Loctite 243) on crank bolts?

It's not the taper that is the problem, it's the arm itself - the profile of the section that sits on the BB taper won't be square any more, and you will need to replace the arm.