An idea for a heavy Scout build, with added long range punch. Mounting the ERPPC and TAG module in the same arm allows the pilot to peek around cover and both target spot and snipe at the same time. I chose an XL300 as it is common to many mechs as well as a popular purchase for many people, though it did mean reducing the Mech's armour rating a little more than I'd like.
I can sort of see your point. However, translating the load-out to a 3M means losing the third MLas. Plus, you would get an overlap of functions if both ECM and BAP are installed, which seems a waste of tonnage. With an XL300 engine, this build is still PDQ, which when combined with the AMS module grants a good degree of safety from enemy missiles. If an enemy Scout does manage to catch you, you'll be thankful of the extra bit of damage from the third MLas as well. It's by no means a perfect build. Its just, in my mind, having an ECM module turns you into something of a target, simply because it is so versatile and therefore such a pain in the arse for the opposing team.
I've been using my X-5 today for a bit of cash grinding - in a few cases I noticed that the absence of ECM can be a bonus, like when you are stalking some targets. With ECM, the 'low emission' warning on the other's LADAR would probably have announced my presence.