Light mechs require an XL engine to move fast enough to be a viable unit so it comes at a price tag equal to about 3 combat vehicles with fuel cell engines . The result the light mech is less than ideal for defensive purposes . Every attacker has to haul it's forces interstellar distances along with it's crew so the attacker might very well not have the option of exchanging the light mech with combat vehicles because most rent drop ships as opposed to buy and tailor the bays for optimal use . As a result the presence of light mechs still happen despite efficacy and economy . Medium mechs tend to be at the break even point ; heavy and assault mechs tend to be a win .
I can't say that I agree:
1) Many 'Mechs can move quite fast even with Standard engines: HER-3S2
Hermes moves 9/14(18), for example.
2) Mobility: Light 'Mechs can be equipped with Jump Jets ...
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Spider can jump over or on high cliffs where your vehicles can't climb no matter what you would do (unless you specifically bring extinct or very rare units).
3) Passability through the terrain
- BattleMech has advantage that it can easily go to many places where vehicles either can't enter at all, or with difficulties - no matter how many vehicles you have. Heavy Woods. Underwater. City landcape. Vacuum of space.
I know about many good Light 'Mechs with standard engines - SDR-8M
Spider, HER-3S1
Hermes, MON-67/70
Mongoose or DRT-4S
Dart, etc.