It is used as a flat currency by some people because it is easier for some to get ectos than platin (e.g. doa gives me 41e/3 runs but maybe 10k if I don't go looting for 20min after the run) so they just offer ecto for stuff like mods and so on.

But for the most part it is still used for trades where platin is not a feasible solution. For example: I buy a year worth of cupcakes which for me (because I use one everytime I leave a town, regardless if it's sc or not) well above 50 stacks. The prices were around 5e/stack which for the sake of simplicity I will equate to 40k. So for my 50 stacks cupcakes I do 8 trades, 7*7 + 1, no problem paying that. If I had to do the same with platin it would take 25 trades which sucks balls and is annoying.

And even that is a trade that would have been possible to do entirely in platin, no problem. But let's have a look at other stuff like rare minipets. The supply of those is pretty damn limited so they have a high value. That's how the market works. So if you want to buy a minipet that is one of only 20 of those in the game or a unique henchman tonic, how will you pay for that without another currency that exceeds the limit of 100k per trade? That's were ectos still come in and after the release of DoA armbraces come in for even more valuable stuff.

In a game were the maximum amount of gold transferrable in a single trade is so limited, other currencies will be found. It has happened in other games that have the same problem but it rarely happens in games where there is no limit in the amount of money you can transfer per trade. For example, I played Aion for a good amount of time and there are some items that go for millions or even billions of kinah (gold in that game). But there is no currency like ecto around (or at least wasn't when I played) because you can just pay the price with your gold. As long as the game limits the players, they will find other currencies.

Btw I just checked and apparently I made a trade in wartower back in 2006 which was worth 100k+15e...