(not a pro): I think the 'overflow' tube is actually a pressure relief tube. So, if the valve on the top pops, you'll get steam out of there and minimal actual liquid water. Discharging into an overflow pan won't do anything in that case.
If your water heater develops a leak, it won't be from that tube. I saw someone put their whole water heater into an overflow pan with a moisture-activated alarm and a valve that shut off the cold h2o supply to the heater in the event of a leak. If he hears hear the alarm, he might be able to drain the residual water through the drain valve (at the bottom) b4 the overflow pan is filled. If he doesn't hear the alarm, then at worst he'll get only a tank's worth of water damage.
Another note: some people say it's good to 'pop' the valve every year to test that it works. Others say not to, because it's a pain to get closed again. Check with someone who knows for sure b4 'testing' it. I haven't havd the guts to try it yet.