FWIW, there is another reason for asking someone to dance on the last song (or next to last song). If I don't like the first song, I'm usually not asking someone to dance. Now every once in a blue moon, a subsequent song will be one I really like, at which point, I'll be quickly trying to find someone to "finish the tanda". Usually, I'll ask someone I know though, and not some newbie, but if that's all that's available and I want to dance, I'm asking.
And I would never turn YOU down... (well, except for milonga, but I turn everyone down for milonga... I even had to turn down a leader in DC who I had been watching all night! I did make it plain that it was only because it was a milonga and he did come back to me the next tanda, thankfully)