Why the fascination with subwoofers?

Excerpt from the OP:

I guess I understand why any audiophile would want to hear more tight bass in their audio system, but why add a subwoofer to an existing audio system when they don’t always perform well, are costly, and are difficult to integrate with the many varied speakers offered. Additionally, why wouldn’t any audiophile first choose a speaker with a well designed bass driver designed, engineered and BUILT INTO that same cabinet? If anyone’s speakers were not giving enough tight bass, why wouldn’t that person sell those speakers and buy a pair that does have tight bass?

... but why add a subwoofer to an existing audio system when they don’t always perform well, are costly, and are difficult to integrate with the many varied speakers offered.

Almost always the objection--"poor performance, costly, and difficult to integrate."

"poor performance" When it comes to sub 100Hz frequencies, the powered subs outperform most "full range" speakers. 

If you check the response curves of most "full range" speakers, their frequency responses start dropping off at around 100 Hz. Sometimes you can compensate with good room reinforcement, but if you're on a wooden suspended floor, forget it.

"difficult to integrate..." Not if you use powered subs with the right controls. My two subs have controls for volume, crossover, and (most importantly for integration) phase. A 2-way 180 deg. toggle won't cut it. Mine have 0-180 deg. phase controls. The controls are continuous with no notches nore 2- or 3-position toggles. The volume integrates the subs' outputs with the mains, and the crossovers match the LF dropoff curves of the mains without a hump or dip where the subs pick up the lower frequency curve.

It takes me about 2-3 hours to integrate subs with my stereo, more likely 1-2 hours. For phase I use a mono recording to adjust each channel separately and make sure the channels' phase blends seamlessly with the mains.

For a final check I use the Stereophile test & demo CD, and listen to the bass frequency samples that drop from around 150-200 Hz on down to about 20 Hz. I generally get a linear dropoff down to around 36 Hz, which is a good match for my Magneplanar 1.7s, which are fantastic in the 80-20K hz range but missing in action below 80 Hz.

"...are costly,..." If you want full range reproduction, powered subs can be a stone cold bargain. Consider the true full range loudspeakers such as Wilson XLF and top line Magico. They are hundreds of thousands of dollars. Now consider a pair of KEF LS50s at $1300. Yeah, their LF limit is 80 Hz, but from that point on up to 28Khz they are Stereophile Class A loudspeakers, all for $1300. Add JL Audio Dominion D110 sub for another $1050 (or two for $2100) and you have a full range S-phile-rated Class A pair of speakers (with anechoic bass extension to 27 Hz) for $2450 to $3400, which is far, far less money than full range floorstanders that could match the KEFs in speed, image, linearity, clarity, and bass integration.

Remember that with full range speakers, the massive deep bass backwaves create a turbulent tempest in the enclosure's interior, interrupting the cone movements of the midranges and possibly the tweeters as well. Powered subs keep all that bass backwave drama in a tight box separated from the mains.
So from my perspective, quality powered subs increase performance, enhance clarity, make room placement easier, *AND* save money.

A lot of the cost of full-range floorstanding speakers goes to the herculanean efforts to keep the cabinet panels and braces from vibrating. Exhibit A: WIlson XLF or whatever has followed.

I use 4 subs but I have floor to ceiling linear arrays and two subs just won't make it. If you are clever and work with your room acoustics and you have a point source system which is just about all of you, 2 subs will work perfectly fine. In most normally sized houses I do not see the need for drivers over 12".  I think 12" is perfect. In addition to a properly designed driver the key to sub performance is a very heavy solid enclosure. Any vibration of the enclosure is distortion. If you put your hand on the enclosure at volume and you feel it buzzing away you have a problem. The best enclosures do not resonate at all and you do not have to do anything to them. If your enclosure vibrates the first thing to do is go to the local granite store and you can usually get cut offs for cheap. Just place an appropriately sized piece right on top of the sub. That can help tighten things up quite a bit. Also make sure the sub has 3 spiked feet not four. Three feet make a much better connection to the floor. No rocking. Subs have to be right up against a wall or in a corner. They are 3 to 9 db more efficient this way.

Some people are after All the response , not just down to what their neighbours are ok with. If i had two speakers that incorporated the subs involved they would weigh 250 pounds each . A four way system tri amped is far and few between and i dont think manufacturers are willing to sit on them hoping someone will buy them before some fancy pants gives them a thumbs down. Everything is a seasoned to taste affair . Pick your components and turn your obtainable mains into monsters for a much more realistic concert experience. Or bump them up a little bit in low listening levels to make the sound fuller .

One benefit of of more subs is the ability to run them all lower . Less buzzing . Less distortion .