We haven't played enough yet to be able to fully answer your question.
One thing we have found that's somewhat significant though, is that little ships are a nice way to up the number of maneuvering elements a fleet has. If you're going to use written movement, then this won't help. If you're going to use the sequential, alternating movement mechanic, then it's nice being able to maneuver some of your ships after your opponent has finished for the turn.
We've also started experimenting with the Escort ability, and having smaller ships with Escort may prove to be fairly effective at screening the bigger ships until they're within range.
I'm sure there'll end up being multiple reasons why you wouldn't want to simply have two or three fairly powerful ships lined up against six or seven that have specific uses or abilities.
Kevin

Either is good. Right now I'm planning to stat out the nsl ships from gzg.

Tell me more.

I have been playing two or three  16to20-hull BB sized ships with shields 4, escorted by 4-hull DDs with shields one.  These DDs have "Laser Cannon" that have no piercing or any other special abilities.  They do have armored hull as do all of my other ships.  The DDs give extra firepower, especially more AA if needed.  All the weapons of the BBs & the DDs have range 18 & 3+ to hit.
I also have made 2-hull torpedo boats with higher speed and shorter-ranged "Torpedos" that do extra hull damage.  They stay at range, even using EM.  Then when the biggger ships are engaged, the rush in and fire their torpedoes.  These torps are "g" arc onlu, but have over thrusters.  I have not built any crusers in a long time.  I like BBs & DDs.  If I use fighters, I use the Long Range ones and don't have any carrier designs.
This usually works; but sometimes I win, sometimes I don't.  Just playing the game using ships I designed is a Great Deal of fun for me.

I think this depends on setting.  In our B5 games we take smaller craft because they are normally faster and more manoeuvrable.  With bigger ships you can end up out of position and arc, particularly when there's a lot of terrain.  So you can use the smaller ships to cover certain "blind spots". 

We've also used them quite effectivley as strike ships to "charge" up the board to attack long range ships that are causing issues.  Ships with energy mines etc.

For comaparison a large ship for us is 500+, we have ships that cost 150 ish that are great for the jobs described above.

In SFO it specifies enemy fire is blocked, last I saw, the new rules do not actually say that it is only enemy fire. I have suggested a little clarification.
It is good that the "little ships" have found another reason to be targeted.

Paul

I think blocking LOS for both makes a lot of sense, actually.  To block off a piece of space as big as a hex, it'd have to throw up some serious ECM, which might reasonably be expected to mung friendly fire as well.  Plus, with alternating movement, this isn't that huge a problem - you just move your escorts last to the places best for you and worst for enemy.  Would be a mess with pre-plotted movement, but that's OK.

In short: I like it!  Now just need to find out what Scout does...

I thought it came up that in the next iteration, Stealth now works like ECM / Countermeasures (generates negative column shifts), except that it isn't damaged by crippling...  Don't know if that's still the case, but if it is, then escort stealth ranging is not a problem.