I have an uber awesome amp, why buy a speaker with a built in woofer sub-woofer amp

It makes some sense to provide a powered woofer for certain kinds of systems.  If the midrange/tweeter assembly is quite efficient, but, the designer cannot find a suitable woofer of equivalent or higher efficiency, a conventional design would mean having to pad down the tweeter/midrange to match with the woofer and that would waste the higher efficiency of the other parts of the system.  With an active crossover and powered woofer, the system owner can use a much lower powered amp for the top end.  

This is a VERY important feature of speakers, like those made by Avantgarde, because very low powered amps, when used with suitably efficient and easy to drive speakers, sound great.  I have not heard ANY solid state or tube amp that sounds better (for my taste), than good low-wattage pushpull or single-ended tube amps.  

I do not like the idea of subs built in to full range speakers and will stay away from them. I do not like the idea of limited subwoofer placement. 

This is a good deal if it suits application.  
https://www.audiogon.com/listings/solid-state-anthony-gallo-acoustics-ref-3sa-sub-amp-2016-04-18-amp...
http://www.roundsound.com/reference-sa-amp.html

I heard some entry level 600 series B&W speakers. The 605, with a sub amp and speakers, was able to produce a greater sound level with a smaller main amp. Also, crossover losses are less. I have some old B&W DM 1400's This speaker was also made with built in amps and sold as the Active 1. It played louder and cleaner than with a Phase Linear 400 on the 14's.

I have main speakers that are 3 db down at 20 hz.  I added a couple of powered subs that go to 16 hz for movie watching, but listen without subs to  just music.

A lot of so called full range speakers do not deal well with bass below ~35 hz, which is fine as long as you don't know it is missing.

My 3rd system has Vandersteen 4s that have separate bass amplification.  I believe Richead went to on board amps in subsequent models to ensure quality of bass (people over driving unsuitable amps for the bass)

While onboard powered subs in a speaker can cause problems in optimal speaker location, their voicing, being that the manufacturer has control of that aspect, trumps the separate sub or subs in some senses. One such instance is the ability to integrate the sub into the overall frequency response pattern, a benefit which answers to some degree the main drawback of adding subs to a system. Cabinet resonance must be dealt with and getting the best ball response and imaging from the same location requires careful placement. In my experience the placement of this type of speaker is even more critical than normal. This was brought to my attention by Jim Smith and I must say he was on the money here. 
Nodes are another factor where having the subs in the same location as the mids/tweeters can cause issues. But to be sure dialing in the correct placement of separate subs can be frustrating as well, not to mention speaker cable placement and ability to have the subs in a user friendly location. 
All in all, I worked it out to my satisfaction and wouldn't change anything. The balance in the spectrum built in by the manufacturer overcomes the downsides, at least in my experience.