It is really useless if you want the best x numbers of units in your lance/company/so on. Using those is only economical, fluffwise and/or taste issue, such as lower C-Bill, lower BV, and/or your own personal interest.
The only exception for this would be, if you want a mech that is able to stand atop of a building and do not want the building to be collapsed. Then you would need for a urbie or its kin.
The other possible exception is when you are GMing a campaign and want to put some expendable weak enemy to be crushed freely by your party.
Also consider that not all lights are equal. Light covers 20 to 35 tons(if you ignore ultralight). A 20 tons mech is not the same light mech with a 35 tons mech, which have almost double of weight.
Personally, I think that it is fine to regard those mech as the alternative protomech, rather than full battlemech. So around three of those slow lights are considered as one full battlemech or combat vehicle in the lance formation, for example.