Even Russia has taken shots at him, implying that Turkey is the means by which IS is able to turn oil into cash and continue operations (my money is on the next US president deposing him).
I just can't understand why Merkel would care what he thinks?
Because Turkey is one of the main passages of immigrants to Europe. And Erdogan can limit the number of immigrants passing EU border. Turkey already got 6 billions Euro from EU to keep immigrants at their country.
In politics two things matter: (a) what you can give me (b) how you can harm me
Of course Erdogan can make further requests because Turkey still have these immigrants.
A side note to the comedian Böhmermann: he did also the "Varoufakis Gate". He is quite a genious satirist with real political influence.
Apparently Europe isn't in control of it's own external borders anymore (in case of Greece this is to be expected of course). And the Schengen treaty removed many internal borders. So now Merkel wants Erdogan to stop refugees on behalf of Europe.
Erdogan gained a lot of indirect power in Europe with the agreement and (IMHO) this is not a good thing.