In some cases learning a new software/service/technology can take more time than its worth. This is not one of those cases. The time it took for me to learn out to effectively use Google Tag Manager (GTM) for the initial purpose of unifying analytics information across different websites (in my case, our website our database/admissions/SaaS) was well worth it. Manually editing the tracking codes on each site may have been the quicker route, but this service enables the editing of a container on a single website that hosts multiple snippets.
Now, with one code insertion, I have 11 tags on our site that includes multiple GA installations, link click tracking, scroll tracking, and more. If we were to add another site, in order to track across domains I would have to insert just one code and not have to edit it in the site code ever again.
While this is most useful for enterprises with dozens of accounts, it allows me to additional tracking features (like event tracking, scrolling) without having to learn the necessary javascript to add the code myself.
It’s telling that Google now lists using GTM as the first option in setting up cross domain tracking. While some would say it primarily shifts the implementation work out of the CMS and into GTM, once a person learns a few macros, rules and maybe some regular expression it can make things like event tracking, adding additional code (ex. AdWords remarketing) and future changes much easier.