Advantages of multiplying drivers?

Audiokinesis pretty much has it nailed. The only thing I would add is that there are some advantages to going MTM rather than TM for a 2-way. It's a set of trade-offs, but with an MTM you have the advantage to splitting the "work" from 2-3K on down among 2 drivers. Since it's really hard to do a good 2-way with mid/woofs larger than 6.5-7 inches, you have a hard time getting a lot of bass SPL's out of them. Adding a second driver allows for more bass, and if you listen at somewhat louder levels, lower distortion and less voice coil heating. That said, it's usually better to go TMW 3-way if you need the bass and SPL's.

Audiokinesis...Remember when cars were sold on the basis of how many cup holders they had? Sometimes I think the proliferation of drivers is the same thing.

It's counter-intuitive, but two horizontally stacked drivers (playing the same signal) will have a narrower radiation pattern in the vertical plane than just one would. The designer can measure or fairly reliably calculate what the pattern will look like at a given frequency and take that into account - the vertical pattern will start to look like a 3-leaf clover or even a bunch of bananas at high frequencies due to lobing.

In general I like TMW's better than MTM's or WMTMW's, because the TMW format tends to have smoother power response - something I place high priority on. However, there are techniques that will give good power response with MTM or WMTMW formats. In my opinion Tyler Acoustics does it right in their Pro Dynamic speakers by using a horn to shape the tweeter's radiation pattern so that it better matches with the vertically-straddling midranges or woofers. That's the way I would do it, anyway.

A good vertically symmetrical design can have reduced floor and ceiling reflections in the midrange (3-leaf clover with the nulls toward the floor and/or ceiling first reflection zones). Typically the floor bounce arrives right at the time delay where the ear is most sensitive to coloration from a reflection (James M. Kates, "A perceptual criterion for loudspeaker evaluation", AES, early 80's), so a null in that direction can help. Unfortunately the frequency range where the ear is most sensitive to coloration (3-4 kHz) is usually still reproduced by a wide-pattern tweeter.

In some cases the decision to go with multiple drivers may not be acoustic. The trend is towards narrower speakers (higher WAF), and multiple small woofers will let you use a narrower baffle. For instance, the Paradigm Studio 100 (I think that's the model number) is a TMWWWW, and its quartet of 6.5" woofers has as much air-moving capability as a single 12" woofer, but shoe-horned into a box barely wide enough for a 6.5.

Duke

Multiple drivers increase the cone area, so that each driver needs to work less hard. This is a step in the direction of a planar speaker. If you want to get the dispersion characteristics identified by Kal you really need a "line array" consisting of eight or more drivers.

Disadvantages include lobing effects when there are multiple drivers that perform in the same audio band.

Better horizontal imaging if the drivers are arranged vertically and if the cabinet width is reduced.

Kal