Is the 2.5 way speaker the ideal home speaker?

@timlub

Everything that Erik has quoted throughout this thread is correct.

Thank you kindly.

@marqmike

This has been a great thread for me to understand a little bit better about something I enjoy. Thanks Eric for the thread

Glad to be of service! What makes these threads interesting is the wide variety of experience and expertise everyone brings to them.

Best,

Erik

As a DIY speaker builder, I've logged many hours building & tweaking crossovers, and one big compromise of a 3-way design is the HP filter you have to put on the midrange.  This is, at the very least a large capacitor to roll off at ~350 Hz, plus a large inductor too for a 2nd order.  These components are now in the signal path of your crucial midrange frequencies, and they're also affecting the phase alignment of the midrange with the tweeter.  You may also need to invert the polarity of the mid driver (or not)... it just depends on other parts of the design.    The 2.5-way design has fewer crossover components in the way of the midrange, and has more low end capability than just a 2-way.  There still will be some phasey issues between the 2 woofers as the lower woofer rolls in to align with the upper woofer.   I admire the idea, but I've never built or owned a 2.5 way design.

@microlab
If you properly account for the resistance of a coil and use quality parts, I’ve not seen a problem with crossing at 350 hz. A simple cap will typically run between 50 and 150 mic’s. An inductor can certainly add phasing problems... using an inductor and porting can cause real phase issues in a woofer, but with proper crossover point selection, using driver compensation and quality parts, most issues can be avoided. I would highly recommend that when you can to just avoid using 12/12 slopes, that will normally stop the need of having to reverse polarity. 6/12 or 12/18 butterworth are both more phase coherent than 12/12 anyway. If you need to run 12/12, choose linkwitz slopes.

Bump start this thread as I have just come across it

My take on 2.5 ways is that the benefits are only if using roughly 5.5" driver size. Plus points- covering 400hz and below is covered by the surface area of 2 drivers. So baritone vocal range can be surprisingly good with 2.5ways. Also as 2 drivers share bass duties the top midbass driver has less work to do (no bsc) compared to a 2 way and mids will benefit. Still, a 3 way is better in this respect.
Also cab size is kept compact. I dont like the term ’WAF’ but a small footprint with good bass output is attractive.
One disadvantage is a sometimes boomy bass which requires bringing the speakers out into the room. Which kind of defeats the purpose of compact and room friendly. This is due to the 6db gain from the bottom woofer. Typical room gain adds around 2db lift at lower frequencies, so the baffle step compensation of adding .5 woofer can often be too much.
Hence surprised we dont see more sealed design 2.5 ways imo.

I think it is helpful to think of speaker design as largely being an evolutionary process based upon consumer demand. There is a reason why large Altec Lansings and Klipsch K-horns gave way to acoustic suspension like the AR3a and KLH to Advent and then to the predominant design of the day-narrow baffled deep floor standers that are ported and either 3 way or 2.5. 
There is a reason too why the outlier designs like planars, wide baffles, open baffles, time arrays, plasmatrons, you name it remain outliers. 
It is largely a matter of being able to deliver the most-looked-for audio attributes for an average consumer's room with an average consumers choice of amplification at a reasonable cost. It is a matter of survival of the fittest. The companies that thrive innovate and evolve and their research and development over time begins to converge on a common solution to these consumer wants. 
In the Preamp/Amp forum someone asked about amplification for the Spendor D7.2 and I tried to help but unwittingly derailed the thread a big when I compared and contrasted the D7.2 with the DeVore O/93's. I own both. I was thinking of this very concept while describing what makes the Spendors so attractive. They do the imaging, soundstage, and midrange things exceedingly well. Most but not all present-day audiophiles want these attributes. Standmounts do these things exceedingly well too at the expense of bass and authority. But again, my main point is that the DeVore O/93's play to a whole different suit of priorities that for better or worse, are sought after by a minority of high-end listeners. I happen to prefer the DeVores but for variety I love the Spendors too.