If you say 'Thanks in advance' then what do you say if someone does come to your aid? Thanks again? Why not just say 'please'?
If anyone actually helps you out you can always thank them afterwards for doing that, in fact they might be pissed off if you don't thank them, which you probably wouldn't because you would already have thanked them in advance.
As an illustration of this point, would it not seem absurd, for example, that a damsel in distress tied to the railway tracks calls out, 'Help, help me, and thanks in advance.'
The thanking in advance implies that the help will be forthcoming, which is a tad arrogant and smacks of self-entitlement, whereas the simple 'please' shows due humility and acknowledges that you are asking, not expecting.
'Pass the salt, and thanks in advance'. Just doesn't cut it, sorry.