I'm definitely a biased Marik fan, but I'd argue the Yeoman is actually a fantastic fire support design for a 3060's FWLM. Allow me to explain.
A lot -- and I mean a lot -- of 3050's fire support 'Mechs mounted Artemis FCS to their LRM racks. This is especially true in the FWL, where even the little Hammer tossed them on to its measly LRM-5s (and for the unaware, yes, the plural is unfortunately intended). The Archer-4M has them. The Apollo has them. The Perseus-A has them. About the only 'Mechs with LRMs in FWLM service that didn't have Artemis systems mounted NARC launchers instead (like the Trebuchet or Orion), but these had a similar issue.
And then, in the late 3050's, the FWLM started using the newly-designed technological terror called the semi-guided long-range missile (SGLRM). SGLRMs get a bunch of bonuses when shot at a target designated by a friendly TAG laser, and unlike Artemis FCS, these bonuses are not turned off if the target is protected by Guardian ECM. The League was already building or importing a bunch of battle armor and 'Mechs with TAG lasers, so the pairing made sense.
However, I'd imagine that Marik planners quickly ran into the issue that you can't mount both an Artemis seeker and a semi-guided seeker on the same missile. So if you're loading SGLRMs into the bins of, say, an ARC-4M, you're now completely wasting two full tons that could be armor or medium lasers or heat sinks.
Enter the Yeoman. It has a higher total throw-weight than any of the other medium or heavy fire support designs it was designed to supplant. It didn't mount Artemis or NARC, so loading SGLRMs did not result in wasted tonnage and thus side-stepped the efficiency-loss issue. The lack of secondary weapons and or super-thick armor wasn't considered an issue because if you're firing SGLRMs, you don't need LOS and thus can safely hide where enemy hunter-killers shouldn't be able to see you.
SGLRMs were originally, I'd imagine, mounted on non-upgraded Succession Wars variants of fire support 'Mechs because the 'Mech itself needed no upgrades to its internal systems. So long as there was a Hermes or some Achileus skulking about, that 'Mech had become more accurate with its missile fire. However, FM:FWL (dated 3058) states the FWLM 'Mech corps has about 70% of its strength fielding post-Helm upgrades due to the rapid re-industrialization that occurred in the wake of the Marik-Davion Outreach pact. By the time the Yeoman entered service in 3060, that upgrade percentage would have been inching even higher. This in turn means that the number of SW-era designs capable of fielding SGLRMs without suffering compatibility issues with Artemis or NARC is rapidly shrinking.
If you're a military planner, seeing that inefficiency in fire support -- and since this is the FWLM we're talking about, bear in mind that LRMs are one of their preferred weapons -- is going to cause some concern. Do you abandon the SGLRM as a technology? Start your refit cycle all over to remove A4/NARC? Or, do you bring into service a fire support platform that is purpose-built to utilize SGLRMs and avoid the mass inefficiency issue? It's my belief that League planners chose Door Number Three and thus we have the Walking BOOM Box, the mobile billboard advertising your death, that is the YMN-6Y Yeoman. And if you need more evidence this was intended, then consider that the Yeoman was built by Curtiss MiliTech. Curtiss MiliTech also manufactured, at about the same time, the EGL-2M Eagle which is, in effect, a jumping TAG system. It's almost as if Curtiss designed the two to work in tandem.
Thank you for coming to my TED Talk.