JMan,
It's certainly not true that a 12-hour totalizer is necessary for a "true" chronograph, but it is true that it's necessary for those uses that require measurement of multile elapsed hours.
Traditional, manual wind chronographs by most of the finest brands have only a 30-minute totalizer, and no Patek Philippe had a 12-hour totalizer until the introduction of Ref. 5960, followed by Ref. 5980. The more recent and exotic Ref. 5708 also has a 12-hour totalizer, but one that is less useful because of the mono-pusher design (i.e., one cannot start, stop, and cumulate elapsed time, but rather must record elapsed time whenever the chronograph is stopped and restarted).
As a chronograph enthusiast and daily user who has little use for the 30-minute versions, I've come to think of the 30-minute chronographs as old-fashioned horse race timers, many of which are gorgeous and superbly made, and 12-hour chronographs as functional tools for working professionals. So far, I've only collected the latter, but this latest posting by the diabolical Vlad has tipped me over the edge to go for the 5070P.
Park



