Ok, it seems that in the Metal Gear games the Metal Gear is some sort of super weapon, but there are several problems with that:
1. ICBMs can already reach pretty much everywhere on the globe, and these are in concealed silos, not a massive walking battle tank.
2. It's would be pretty much ineffective in combat, as a 120/125mm tank shell to an important part of the leg would give it a very bad day.
3. Following up on that, the Metal Gear is a pipe dream for CAS.
So yeah, why the hell is the Metal Gear potrayed as some super-ultra weapon?
Basically, because they're easily controllable, mass-produced, can launch undetectable nukes, have almost indestructible armor (from usual weapons), and are really the combination of super-mechs and nuke silos. Yeah, it's pretty impractical, but I think the main reason is because all you have to do is hop in the cockpit, and boosh, walking battle tank. Plus, it's really fast, mobile, and agile, as proven in the fight between RAY and REX, while other missile silos are stationary.
Metal Gear, in basic concept, is the land-based equivalent of a ballistic missile submarine. Its mobile nature means that unlike fixed silos, which can be easily targetted in a first strike, and truck-mounted launchers, which require roads that, again, could be damaged in a first strike, Metal Gear could theoretically launch from anywhere on land, thus guaranteeing so-called "Second Strike" capability, or the ability to retaliate after the user's nation has been struck with nuclear weapons, even if all "First Strike" weapons have been knocked off-line. Its mobile nature also means that, without an opposing conventional force in the general proximity, it's highly unlikely that they could be targetted and eliminated by a first strike. The result is a near-perfect deterence, the only limiting factor being the reluctance of most human beings to commit nuclear genocide, even as retaliation for the same, which is where the whole concept of Peace Walker came into play, but that's a whole different can of worms.
In short, they're walking nuclear death mobiles which are nearly indestructible and unbeatable in combat(unless your Snake,Raiden or a cyborg ninja) to regular infantry,AFVs and other forms artillery. So unless your gonna launch another nuke at it from a distance in the middle of the battlefield they're demonic weapons of destruction plus they can be mass produced and are highly portable but as you said Metal Gears get a decline in use and battle efficeincy because the cost and supplies to actually make one and also newer tech( like SOP and cyborg technology). The short answer is they can launch nukes ANYWHERE in the entire world and they're invisible to radars and so cannot be intercepted so once it launches your finished!
What 166 said. The series goes to some pains to justify the damned things; all three of the prequel games showcase the concept in a different stage of development.
1964 - Shagohod: A weapon designed to replace stationary missile silos. It uses a road or runway to build up speed to lob an ICBM.
1970 - ICBMG: Sort of an ICBMIRV. At this point, the legs that will define later MGs are no more than landing struts.
1974 - Peace Walker: The first properly-legged vehicle, intended to be capable of traversing the extremely varied and rugged terrain of Central America. There's a lot of NASA rover DNA in this design. I'm not sure how far these things are supposed to be able to throw missiles.
By this point, we can finally justify:
1974, 1995, 1999 - Metal Gear ZEKE and Metal Gear D: The archetypical Metal Gear; a mobile nuclear launch platform. The small size makes it preferable to silos for small outfits like MSF. D is basically a direct upgrade from ZEKE.
2005 - Metal Gear REX: I'm not actually sure what the U.S. wanted from this design. It's possible that it was essentially just supposed to be the concept of Metal Gear D, but with the development budget of a real country behind it. Aside: Big Boss had to invent a giant helicopter to move D around; I have no idea how the army was planning on transporting REX.
In Metal Gear Solid 2, the Metal Gear lineage diverges into two:
2007 - Metal Gear RAY: At this point, "Metal Gear" no longer refers to a nuclear weapon, but to any tank with legs; RAY is a weapon designed to actually engage enemy forces as anything other than a last resort, and it doesn't carry a nuclear payload. It appears that by this point, the technology behind legs is good enough that you no longer need to come up with pages of justification to put them on a tank without anyone looking at you funny - conventional warfare after this point will see increased emphasis on UMGs, which are basically RAY in different sizes.
And, on the side of nuclear weapons development:
2009 - Arsenal Gear: A weapon designed to launch nukes at the Internet. What a time to be alive.
Because silos take months to be completed, maybe a year before they go active. Metal Gear can be mass-produced and fired from anywhere. The enemy is closing in on your base? Just put aside the rail gun and shoot the muthafuckas with machine guns, go to the next area, and launch a fuckity nuke. Also giant robots, bitch.
In a way, the Metal Gear weapon could at its heart be a way to encapsulate the themes of the Metal Gear series, such as the threat of nuclear terrorism, in a form that can be beaten up by one dude. REX, for example, could be said to represent Metal Gear Solid's themes of government backroom dealing and betrayal (it was a secret government project to sidestep nuclear disarmament treaties) in a form that you could blow up with Stinger missiles. Metal Gear Solid 2's themes of computer-controlled, reproducable soldiers are echoed in the army of AI-controlled, mass-produced RAYs, which you then also blow up.
I guess the Shagohod was built by someone who supposedly defected and yet ultimately ended up only helping their mother country, like the Boss, but I dunno, that's kind of flimsy. I'm sorta running out of steam on these.
And Metal Gear Solid 4 just reused the characters and concepts from the previous games, so it didn't have a new Metal Gear; it just recycled two of the old ones.