First, what is a 2.5 way? It is a speaker with 3 drivers, but the mid
range lacks a high pass filter, so it shares output with the woofer.
It has a number of advantages over smaller and larger speakers:
I've always seem this as a poor implementation. I get the idea- the mid bass/midrange driver goes down pretty low, so if you crossed it over, the crossover would impose a coloration due to the size of the capacitor(s) used.
But in practice they are problematic. Of course from my perspective this nearly always means is a bad load in the bass - meaning a lower impedance so more distortion from any amplifier. But further, you can have doppler effect distortions from the mid range unit as it can have a bit of excursion, unless carefully damped by a sealed box, but that does not solve the problem of the voice coil having to absorb a considerable amount of power!
Now for the anecdote: YG Acoustics was advertising at RMAF a few years back that they made the 'best speaker in the world' bar none. I went to audition their speaker at RMAF; I brought with me Peter Gabriel's 'Rabbit Proof Fence/Long Journey Home' soundtrack. In the first 30 seconds or so is a fairly large bass note. I played it at a level that I can play easily at home on my Classic Audio Loudspeakers. When the bass note hit, the mid/bass driver rattled quite loudly! against that wire cover they placed over the driver. I think the salesman thought it was some sort of synthesizer midrange note played loudly but it was really something that should have shook the room. I can name plenty of speakers that don't do this.
So I don't buy that this as a good technique at all. I've always viewed this approach as a bit sloppy and I don't think I'm going to be easily convinced of otherwise.