I don't see why you'd run out of batteries in the middle of something. Any Logitech mouse with non-replaceable batteries last 5-14 days a charge, so leave it charged overnight if you really think you need it the next day. >_>
Call it a hold over from the old days when rechargables have memory effect. I usually don't put the mouse on the charger every night and will only put it on when the battery is low (which is mostly ok since you still have a few hours after low battery warning). The only problem is if you then forget to put it on the charger, you'll probably have a dead mouse the next day.
Even nowadays with Li-Ion packs. I found that having to remember to put the mouse on the charger is more hassle than just swapping pack when needed on the G7.