- The primary attribution model in Google Analytics is last non-direct visit prior to <conversion/goal/significant event>. Leveraging the source tracking parameters (utm_*) on internal links obliterates the usefulness of much of the built in attribution-related reporting. Relatively recent enhancements do provide visibility into multi-channel funnel flows, but the capability is compartmentalized into a few purpose-built views and don't filter back to most of the core reporting.

- It also drastically skews engagement-related reporting, overall and at a source level. A new session in GA constitutes a completely new visit. So metrics like visits per user, new vs existing users, average visit duration, pages per visit, etc all get whacked in the process. And understanding user behavior and conversion paths from any given channel becomes somewhere between torturous and impossible, as the actions performed in the "new" session aren't visible when looking at the user flow of the original session.

What you're describing is possible and commonly done, you just can't do it with the utm parameters intended for source tracking without having disastrous side effects. Custom dimensions/metrics[1] or event hits[2] are the intended ways of doing that. For developers, Google's autotrack[3] library is useful for implementing that with fairly minimal dev overhead (mainly, seeding your markup with relevant data- attributes so event hits can be auto-populated and sent). There are also a lot of third party libraries for framework-specific integrations. And for non-developers, Google Tag Manager (or any other tag manager) makes event tracking pretty straightforward to implement without dev coordination.

[1] https://support.google.com/analytics/answer/2709828?hl=en

[2] https://support.google.com/analytics/answer/1033068?hl=en

[3] https://github.com/googleanalytics/autotrack/#overview