You guys are focusing too much on extremes, while yes, those will exist, this will mainly help people who are not extremes.
What I mean by extreme is a player who gets 0 Eso one week and would then "have to" farm 900 the other week.

The more common situation is that a person has time to do roulettes for example 3-4 times in one week and has no time that weekend. And then next week they have time to do it 5-6 times and have a free Saturday for example.

To use myself as an example:
I over-capped on Sunday on the first week by just doing roulettes (Expert and Trial) every day. I had free time on Saturday and could have done a few more expert dungeons if it was needed.
The second week I only gained ~290 because I had only 3 free evenings and no time during the weekend.
This week I will miss at most 2 evenings of roulettes, and will have a free Saturday (and maybe even Sunday) sou I could EASILY catch up with the 160 from last week.

You see where I am going with this?
There is absolutely no way a cumulative cap would hurt the game or frequent players, but it would help infrequent players immensely. So yeah, it's a win-win situation.