ECM isn't some black and white difference. There are two main effects in jamming mode (the default)- the first, when you're in range of a friendly ECM reduces enemy sensor range and increases enemy LRM spread. The second, when an enemy mech or vehicle is close to the ECM-equipped mech, they get an accuracy penalty - you can tell because there's an extra icon by an affected target. It's certainly nice to have, but with how few mechs can even carry any EW gear, it's not something to really rely on - and even then late game it becomes far more dangerous to be at close range, negating the biggest boost from ECM. Though through the early to mid game at least, a brawler with an ECM slot is a nice addition.

The othe ECM mode simply reduces most of the effects of an enemy mech's jamming ECM (HUD distortion for the player, accuracy debuff for the AI)

As for the Raven, it's a rather poor mech - it's slow for a light mech and for them, speed is life. Further, the EW-capable variants have extremely light weapon loads. The hero variant is slightly better, but has no EW slots.

It isn't exactly a huge benefit and you need to be relatively close for it to impact the enemy to notice it. Having a bunch though in a lance seems to produce some interesting results. May want to look for better Mechs to mount it on though.
https://docs.google.com/spreadsheets/d/15fQCjkxjECs3Jvhv-YA1ZP147SuEP2olkPzLJCLAFmQ/edit?usp=sharing

I often run the Locus Hero Variant (LCT-PB) even on higher difficulty missions. ECMs don't prevent the enemy from targeting you however it may take longer for them to fire LRMs (with increased spread) and at close range (where a harrasser should be) it lowers their accuracy.

A light mech NEEDS speed and decent piloting to be able to dodge and close the distance while either pulling aggro away from your lance or flanking an enemy heavy mech to target the rear armor. The ECM gives a small boost to your survivability while you're attempting to achieve that, but you can still easily catch at nasty PPC or AC/20 if you're not careful.