The answer to your question of "Worth it to transfer CC points to an Alliance to book a DIFFERENT airline?", at least for us, is a very enthusiastic YES!

Indeed, at least thus far, we have *never* used our AAdvantage points on American Airlines.
Our other primary program, thus far (although we are adding some others) is Amex Membership Rewards, and obviously they are not an airline themselves. But as someone already pointed out above, we almost always transfer those points to one airline, but then use those points to book award travel on a different partner airline.

For us, it's not always the "least number of points" that leads our decision.
We care a great deal about the routing and number of connection, and also we have some preferences of airlines, in terms of comfort (at our age, that matters more than it did some time ago).

And also yes, we do use an awards booking service, which has been invaluable. Mostly, they can find routings/airlines that sometimes we wouldn't have thought of, and in some cases cobble together a combination that "works best" for our particular preferences.

At this time, we are only using our points for international premium (J or F) seats. Anything else, we just pay cash, as that wouldn't be a good "value" for the points needed.

GC