You say request so I guess you are talking about a network service request. When I first got into A/B testing many years ago we found we could use it to simulate traffic through the entirety of the Travelocity web site from homepage to checkout and back. They never used the A/B test system for test automation, but they could have. Back then the checkout pages were rarely tested with valid feature tests because they were at the end of the path. There were plenty of unit tests isolated to small bits of functionality at those pages. We discovered, thanks to session recording with things like Tea Leaf and Clicktale, just a bit later that those unit tests were worthless. The checkout pages failed all the time and cost the company tremendously in lost revenue. Some of those failures would show up in our A/B test experiments though.
My lesson learned from this is that testing things in isolation isn't reliable and provides false confidence. You tend to get back the answer you wanted such that is amounts to a self-fulling prophecy opposed to a criticism.
Rather, I'm arguing against feature tests to the exclusion of unit tests. Both are required in a decently tested systems.
Feature tests provide the assurance that the application works as expected and prevent broken changes going to production, as you've pointed out.
However, they're not a practical replacement for the instant feedback that unit tests provide. Our feature tests currently take 1.5->2.5 hours to run depending on CI system load.
That's... not great. On a given workday I get to try 3-4 builds? I don't know about how others work, but personally I like to get some feedback every like 10-25 lines of code. Unit tests provide that. They take 2 minutes to run. If I had to wait 1.5 hours every 25 lines of code, I would sure be much less productive.
Lets say I somehow convinced management to pay for 20x as many compute resources for CI so that I could get those results in 5-10 minutes rather than 1.5->2.5 hours. And 5-10 is the best case, as that's pretty close to the overhead for our CI system to just clone the code and get the secrets to deploy to a test environment and actually deploy to said test environment.
That's still my flow broken every time I need a test run, because I'm not going to sit and stare at build for 10 minutes, I'm going to answer that email from my product manager or review a team member's PR. If that build comes back with a failure, then I'm context switching back to actually fix it.
The point is that this is a funnel. The unit tests are for instant feedback and to catch obvious mistakes, like "Hey, that value might not be defined, you still need to calculate value B, hope you didn't remove/break the fallback calculation". This ensures that less mistakes incur the full 2.5 hour penalty of a CI build that ultimately fails a test. The feature tests are there for safety. The unit tests are there for productivity. The feature tests are "Does this feature work so we can deploy it". In theory you have at most 1-2 runs of these per feature as unit tests _should_ catch issues before they get there. They're simply too expensive in terms of time to be your first layer of defense, rather they're your final layer.
Real users don't use every feature every time they perform an action.
A test case at the feature level will likely require several requests if its starting from clean state.