Well, you said I should post my thoughts, so here goes!
I first off, have been playing as a Force Fields / Psychic Defender since I first started playing City of Heroes back in July of 2004. I have played on and off, but she is still my most played (even at 50) and favorite character with a nice background and good story line (I think its good, although the maximum character limit of the I.D. section forced me to end it with a cliff-hanger). I, in my personal experience, have never found my force fields and powers as a defender to be useless to any team, however they are pretty useless to me if I were to solo. If I were to take a good guess at how often I played solo versus the time I spent in teams (almost always pick-up groups until my later levels when I found a splendid SuperGroup). I'd say that less than 10% of my time was solo, 50% of my time in teams, and the other 40% of the time spent hopping around random zones buffing random heroes until I could get on a team (lol, I really appreciated my SuperGroups' constant desire for me on their team, feeling needed or wanted is a good feeling I believe). Amaylia has changed many times through out her time, but I still feel as though she is as effective as ever and I do share the desire by the devs that every hero have its weakness. I am proud to be a defense specialist, just as proud as I am of all the other defense specialists for sticking out the good fight and showing all the defense "nay" say'ers not to underestimate us!
Anyhow, that is how I feel about my character and my powersets and I'll show my build in case anyone is curious about it.
Amaylia - Level 50 Force Fields / Psychic Blast Defender
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Force Fields
+ Deflection Shield - 5xDefense
+ Personal Force Field - 1xRech Red, 2xDefense
+ Insulation Shield - 1xEnd Red, 4xDefense
+ Dispersion Bubble - 1xEnd Red, 4xDefense
+ Detention Field - 3xRech Red, 1xAccuracy
+ Force Bubble - 3xEnd Red
Psychic Blast
+ Mental Blast - 1xAccuracy
+ Subdue - 1x(Dmg&Mez), 2xDamage, 1xAccuracy, 1xEnd Red, 1xRech Red
+ Will Domination - 1x(Dmg&Mez), 1x(Accuracy&Mez), 2xDamage, 1xAccuracy, 1xRech Red
Power Pools
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Teleportation
+ Recall Friend - 1xRech Red
+ Teleport - 1xEnd Red, 3x Range
Leadership
+ Manuevers - 1xEnd Red, 4xDefense
+ Assault - 1xEnd Red
+ Tactics - 1xEnd Red, 4xToHit Buffs
+ Vengeance - 2xToHit Buffs, 4xDefense
Fitness
+ Swift - 1xRun speed
+ Health - 3xHeal
+ Stamina - 3xEnd Modification
Concealment
+ Stealth - 1xEnd Red, 3xDefense
+ Grant Invisibility - 1xRech Red, 1xEnd Red, 3xDefense
+ Invibility - 1xEnd Red, 1xDefense
Ancillary Power Pool
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Power Mastery
+ Power Build Up - 5xRech Red
+ Conserve Power - 5xRech Red
+ Temporary Invunerability - 3xDamage Resistance
On an end note, I do know about Enhancement Diversification, but I honestly have a ridiculous amount of slots to throw around and placed them where I felt it mattered the most. There are many times I wish I had Repulsion Field, and Repulsion Bomb but I simply did not want to sacrifice any power in my set. I once had Medicine as a power pool instead of Concealment but I felt medicine really threw off my character concept of Amaylia being a sorceress specialized in defensive and evasive magics. I feel I realize all of the strengths and weaknesses of my character and because of that I am able to play her effectively in almost any situation. However, I still have massive problems with endurance given all of the toggles I run if I begin attacking after having buffed a whole team. For that reason I have few attacks, the ones I picked had a degree of control to them, subdue giving immobilzation, and Will Domination giving Sleep, I slotted them up so they pack a minor punch along with the status effects! Power Boost is an excellent power, I use it right before I cast my force fields for what I like to call "Super Bubbles", I can cast 5 super shields before the effect collapses (its very close to being able to get 6, but it simply does not quite make it in time to receive the benefit). I cast these on different people depending on the situation. If some of the squishier heroes are drawing too much attention frequently I will focus 2 super bubbles and a super invis on them to insure their safety, but generally I cast the super bubbles on tanks and scrappers since they are usually at the most risk and benefit the most from my added protection. I invis everyone excluding our lead tank (if we have a tank, otherwise I invis everyone) except if the tank truly needs some extra +DEF all. I'm not absolutely sure if the invisibility truly makes their job at keeping aggro much harder, but some of them REALLY don't like being invisible so I submit to that.
In summary, I think defense is still a highly viable source of protection and is well complimented by minor amounts of healing as necessary. However it is totally useful and dependable on its own given the team has an idea of what they are and are not capable of. Although I do admit that sometimes things go really wrong, really fast just by sheer bad luck, for those times... VENGEANCE! I almost feel like a vengeance affected team is similar to a team of eluded scrappers! But thats just an opinion of an observation.
This guide is totally awesome and enlightening though, congratulations on a guide well done Arcana!