I think the ER chuck is primarily a device for automatic machines where the tooling is set up away from the machine, it has found a place as a very useful general purpose cutter and work holder although these are probably not its original purpose. Changing cutters while leaving the chuck mounted in the spindle needs either a two spanner approach or a substantial spindle lock capable of coping with the recommended tightening torque of the collet nut. Most of us probably give it a good pull and leave it at that and most of the time it will be ok, when our good pull on the standard length spanner is our best effort but not enough then trouble may be on its way. Most spanner’s are of a length that a man of average strength will not over tighten the job, of course there are people with gorilla strength and people who need more time with their Bullworker, I suspect that a professional tool setter will use a torque tool or experience to tighten a collet. The ER collet having a range of perhaps 1mm may be asked to grip a 6mm cutter in a 6-7 collet, this is not the most effective option for maximum holding power. When the optimum conditions are met the ER system has phenomenal grip and a cutter moving even under the most arduous conditions is unlikely, failure to meet the recommended parameters leaves a risk of failure.
Mike