Eh... Not really... It's pretty easy to generate a unique image url per customer and use that to determine whether a customer opened the mail or not and it's about the same cost as a traditional tracking pixel, complexity-wise.
While their cache is large, it's not infinite. If what you're interested in is whether a customer engages with an email multiple times over a few days, you'll likely get the pixel hits to confirm it over that time period. Of course, as you imply, you wouldn't be able to collect how many times an hour a single customer has viewed an email sent to them.
You get enough good information. The IP address would be even better however.
The catch is that Gmail caches all the same images from the same servers, so email marketers get around it by serving tracking pixels with obscure, unique URLs per individual.
Geotargeting fails because it loads on GMail servers.