Obviously Polar finally listened to all the bike riders out there, and fixed this major ovesight with the new S710i series. The auto shutdown time is now 30 minutes, and this I find to be quite ok.
For those with the older S710, the new PPP4 software does allow for merging of HRM files. There are still problems sending files by email (ie does not send the joined HRM files).
I actually had a S710 for some time, but got rid of it for just this reason - I kept on losing too many rides. And you cannot wear it on your wrist (I find). The temperature readings will be totally stuffed, so of course will be the altitude. I also found that you get intermittent pickup from the speed and cadence sensors - especially if you move your arms around a bit (ie, stretch or something).
And there is no way you will remember to take it off the bike when you stop - there are always times you are busy doing something else and forget.