You say marketing gimmick, others who can maximize or at least appreciate the bigger working table will say otherwise.

So I'll try to keep things simple... It's all back to the Super Car vs say an Axia. Both take you from point A to B differently. One clearly has more than enough power to chug along effortlessly. Whilst the other may or may not be enough. Who needs what, depends on the driver's needs.
There are RAM and there are high quality RAM. High quality RAM can be quite expensive. The Nvidia Shield sports 3GB RAM cuz they don't want to be caught with their pants down shipping with 2GB RAM. Why need more RAM? This is a case by case basis. Some or shall I say most can get away with just 2GB RAM and others who do more serious stuffs.. Such as high graphics gaming, whilst simultaneously running a pseudo-NAS and or torrents or home server, etc in the background will benefit with having higher than 2GB RAM. Heck, I prefer setting up as much free RAM as I can to run Kodi smoothly. Caching/buffering high bitrates videos using memory equals smoother uninterrupted playback. These are just examples of use case scenarios where one can exceed 2GB RAM usage on Android.
Android is pretty efficient where RAM management is concerned. However, other factors come into play. You need a decent firmware, RAM and processor. These in combination makes all the difference in launching apps, keeping more in a "saved state" or running, or exiting and opening/switching faster between apps.
Summarily, can 2GB RAM be enough? Yes. But just because it can, doesn't mean one can't do better with more than 2GB RAM. The Nvidia Shield with their excellent processor could have shipped with only a high quality 2GB RAM. But they didn't. I assume they knew and or pictured scenarios where their users will be limited by insufficient RAM. Who knows, the next one might just be equipped with at least 4GB RAM. Like phones of today, bet 5 years ago.. We didn't think we'd be seeing or needing phones with 12GB RAM. I'm sure a ton of people today will still argue that it's useless and 3GB RAM or less is more than enough.. But hey, here we are. Granted a TV Box and a phone are different beasts. But you get my point. Will I ever use up the entire 4GB and want more in a TV Box? High chance, nope. But I know for certain, I fill up 2GB and cry for more very easily. So yeah, contrary to popular beliefs, I'd rather have more than enough than just enough or not enough.
Here are some hidden facts.
1)The TV boxes out in the market currently despite having 64-bit capable processors are limited to only 32-bit wide system bus.
2)Also they are running on 32-bit Android OS with 32-bit apps.
These are the bottlenecks restricting the current design.Also it is very rare to find 64-bit based Android TV apps that could utilize more than 2GB of RAM.
Games probably but if I wanted to play games, I'd either choose a high performance game console or a gaming PC.