While a lance of 55 ton mediums (or their later era functional equivalents) might be the most "effective" escort, it's not a cheap one. A lance of fairly hard-hitting vehicles with moderate or high speed (5/8 or faster) come in at a tenth the cost, and a pair of decent Light 'Mechs at half the price. The lights can scout ahead or race out to deal with fast hovers or other lights, and the vehicles can guard the flanks and threaten anything that tries to close before it reaches weapon range to the main advance. A medium 'Mech would be a nice but more expensive addition, or a sub for one of the lights, but that boosts the cost considerably.
If I'm given an assignment to slow the advance of some group of Assault 'Mechs, but not enough firepower to actually engage them and force them to halt the advance, then I'm probably going to either race in, take a shot or two to force them to turn and deal with me, and then get out before they can concentrate fire on me, or else set up some kind of ambush. With scout 'Mechs or a light vehicle leading the advance, and a couple of vehicles on each flank taking shots as I close AND as I try to escape, it's likely to get REAL expensive for me in a hurry. If I try taking out the escort first, I'm not slowing the main group until that's done, and the escort I'm picking on will likely call in some of the other members of its lance to make my task more difficult. Meanwhile, the Assaults just keep plodding forward, taking a few shots to assist if anything gets too close.
As a defender trying to halt the assault group, if I'm given enough firepower to actually deal with all of the escorts, then it's probably worth the attacker's time to crush my force with their Assaults while I'm engaging the escorts. Yes, I've slowed the assault group, but at an unaffordable price. I would probably have been more effective if I had joined the defenders at the destination, rather than being taken out piecemeal while buying only a few seconds or minutes of extra time.
A Manticore or couple of Hetzers running parallel to the advance won't stop an LRM Carrier from lobbing LRMs over the nearby hill, but a Locust or Pegasus can race around the hill and threaten the LRM Carrier, OR chase down the spotter. On the other hand, the Locust won't stop a Firestarter from racing in to backstab the trailing Assault, but the Hetzer following behind might ultimately make it a losing proposition. Depending on whether the scenario is being tailored around money, BV, or in-universe forces, the actual "right answer" will vary.
The escort vehicles can still provide meaningful fire support for the eventual attack when the group reaches its destination, while the scout 'Mechs will still be able to shield the main attack from anything trying to interfere from behind or to the flanks, as well as to either block or assist escape if needed. It's not like the escorts get "disposed of" when the assault group arrives. More likely, once the assault is complete, they get reassigned back to wherever they were pulled from.
Basically, the escort group needs at least a few fast maneuver units, and at least one or two units with longer-ranged weapons to hold opposing snipers at a safe distance. Vehicles are far cheaper in that role, and easier to obtain in most cases, and should be adequate for at least PART of the task. In most cases, a combined arms approach can beat an "all"-anything.