Sounds a tad weird. If the magenta line's there why not let the automatics follow it on its own? If it does it correct that is, there was a known bug in one of the 737 FMC updates that calculated and traced incorrect holding patterns which surprised you halfway through straight on the line with the message UNABLE HOLD AIRSPACE.
So crosschecking it with timing and a tuned navaid (if the fix is based on one) is still a mighty good idea.