Hello Folks,
(Without saying too much about Google's internal systems,) If you've
ever edited an ad via the AdWords web interface and you're
particularly inquisitive, you might notice that the ad's underlying ID
changes after the update. You also might notice that there's a running
count of how many "edited or deleted" ads there are in the Ad
Variations tab of a given ad group in the web interface. One might
draw the conclusion from this that when you edit an ad's text via the
AdWords web interface, you're effectively creating a new ad and
disabling/deleting the previous ad, allowing our system to keep track
of the previous ad versions with a unique ID as well as the current
one.
Given such a potential backend setup, if updateAds() allowed
modifying ad content it would just be a thin wrapper over what
everyone can already do via the API, namely creating a new ad and
disabling the old.
Hopefully that sheds some (hypothetical) light on why the API might
be designed in such a manner.
Cheers,
-Jeff Posnick, AdWords API Team