For the LAMS, I do not really see them as combat units i nand of themselves. I see them as the ultimate recon unit. Versatile, as in they can be used in a pinch anywhere if needed, but tbh, a unit that can go to ground ,and later break for space without the need of inherent transport does offer some possibilities, especially with advanced sensors, mapping, comms, etc. Such a small unit(A lance here or there) makes insertion, and has no need for a dropship to hit dirt in order to GTFO. Yes, an escape plan and use of La Grange points may be necessary, and the ride wont be comfortable, but they have their place if utilized correctly. The problem seems to be that noone wants to utilize them correctly. In an age with ECM and stealth armor and such during the 3060s + there seems to be lacking the specialized units that, under fluff pretenses, exist simply to get in, get out, and have the capability to self extract like a LAM gives.....
Again, specialized role, versatile hardware. The LAM can, if necessary, engage lesser vehicles. It's not expected that a 25 ton LAM, or lance thereof, goes head to head with a heavy lance, but in that case they shoot and scoot, at incredibly high velocity, while painting a planet's defenses and giving the potential invaders, or raiders, a look at what is nearby without landing a sizable force. Now if only a LAM could have a Battle Armor troop bay...
Now, to say that boots on the ground ,and the specialized recon/black ops units do the same things with less expensive equipment is true. It's just the only really viable use I could find for them, as they are FAR too frail to be used as a front line unit one way or the other. And as militaries are prone to do, if they make that strategic plan come together and work, using LAMS, they'll develop a doctrine around them. Plus there's the rule of cool to consider.