There is nothing wrong with using the "bulb" this way as when you turn on the key the light should be illuminated and if it isn't you uknow you have some troubleshooting to do. A lot of European vehicles with Bosch charging systems, and even those with electrics inspired by the "Prince of Darkness", use a bulb in the rotor excitation circuit.
I went through the same thing with a 1969 MGBGT. The generator light had been dimly glowing for quite some time but the battery always remained charged so, being lazy, I ignored the problem. One day the gen light was not on anymore so I was happy and thought it had "fixed" itself. Three days later I got out of school at about 22:30, went out, and "CLICK"!!! Barbi had to jump me off about 4 times to get it home. We'd stop, hook up cables and let charge for a few minutes, start, drive 5 miles, repeat...
This reminds me of an old joke....
My first car was a 1971 Triumph Spitfire....being british it was riddled with oddball electrical problems.
I went to the local sports car guru...walked in his shop and said "Bill, I'm having terrible problems with my Spitfires electrical system"
All the mechanics working there giggled and Bill the owner of the shop said:
"Do you know why the British drink warm beer? It's because Lucas (maker of my wiring harness) makes refrigerators too......