I think if you're waiting for them to actually start to change langes before responding, then you're leaving too much up to chance. Because if you go to pass them they might start to merge when you no longer have any safe options available - either because they didn't see you, they misjudged, they're just an a-hole, etc. There are a lot of unpredictable drivers out there.

I think the safest thing - and the most polite - is when they first start to signal, make a determination if you are able to safely give them space by easing off the gas and maybe lightly tapping the brakes (of course you don't want to do any hard braking because that creates a risk of being rear-ended). And if the answer is yes, then let them merge. If you're leaving a safe distance to the car in front of you, and assuming they signal about the time the car in front of you passes them, this should be doable more often than not.