Totally hear ya, but the concern is premature failure of the "new bulb". Not only does there appear to be lots of complaints about early RS40 bulb failures, but at least in my own limited experience (with my DLP TV) a projection bulb that's going to go bad far-earlier-than-expected, tends to do so in the first 6 months (assuming moderate use). Buying expensive spare bulbs and leaving them on the shelf for a long time means when you finally go to use the new bulb that's been sitting on your shelf, it could fail in days, weeks, or a few months, and you're out a lot of $$ because the warranty already expired.
That's why I'm interested to find out why JVC is warning against re-using your old bulb and claiming it can cause a malfunction. If the warning isn't something to take seriously, then ideally you pull and shelf the old (working) bulb to be your "standby", and run the new bulb under warranty.