I agree that Def/HP champs that can deal damage are great because of the reasons you mentioned. 

However, I can't dismiss the Attack champs as entirely inferior. You'd still want your Royal Guards, and Coldhearts for dungeon clearing. You'd still want attack champs like Trunda, Skullcrown, and Foli, as nukes in arena. Even everybody's favorite Kael is an Attack champ. 

Every champ type has a role. Some def champs are better than attack champs, and some attack champs are better than def champs.  It all depends on what you need them for. 

(Even the Frozen Banshee you mentioned on your example is an Attack champ, and of course Saito is never gonna replace her for CB)

I guess I'm talking more about raw damage attack champs, poisoners are always going to be useful. It's just a shame that I can nuke just as hard with a well geared Tayrel as I can with a well geared attack champ, arena, dungeons, anywhere. For me any single target attack champ that brings no group buffs is a useless champion, and when that champion is legendary it becomes very painful to see them running out of a shard. 

Trunda  (Dwarf)

Attack base and most would argue one of the hardest hitting Legends  in the game

just 1 example

would not beat my Tayrel 1 on 1

And she would only get 1 shot before being killed. So how useful is she? 

This is not a 1-on-1 game.  And if Trunda goes 1st... I never fear a Tayrel, but a Trunda can clean wipe a solid defensive team.  I am pretty sure damage is significantly higher than Tayrel.  Note, I own neither :(

Your question/assumptions have multiple flaws:

1. I generally ignore the game designation of champ as atk/sup/hp/def and build according to skills.  Alure is Atk, build as support for example.  You clearly understand this as you recognize poisoner is a category in it's own right.  Banshee an atk champ? not really.  Brawler an HP champ?

2. You don't need atk champs to clear any game contemt.  I'm fairly certain I completed arbiter missions on my 2nd account without using an atk champ.

3. Your assumed mix includes 2 Atk champs, this is a problem many believe as it is great for first 10-12 levels of dungeons.  Then you are stuck with a bunch of useless 60s (sorry Elhain and Athel and even Kael are relegated to FW & campaign farming).  Attack champs are geat for very early game, and then again late game when you have 1-2 supports that are great enough to do the same as 3-4.  Until then, the majority of the mid-game you need 0 to 1 atk champs for most content.

I got Trunda and can vouch she is brilliant, everywhere

one on one she can defeat tayrel

teamed up with a good arena team , she can win hands down. but I have seen teams defeat my team even though they had less powerful teams. various factors come play, great hall bonus, masteries, arena bonus, artifacts, accesories. 

Infact I recently lost to an army of Spirit host, Kael, Warmaiden and one other champion , my team was dead before my turn came . I replayed it and it turned out that the Spirit host went first and then Kael went twice and finished my team flat out.

i

I hear you and agree. 

I have Tundra in my arena team. My arena team is power 187,000 power all legend

She is my damage dealer and I tried  others which give  me more total power but she is the match for the team "I built"

However I still get wrecked from time to time from teams  with much less power and  no legends. I can only attribute this to them haveing a greater understanding  of this game than I

But I just wanted to point  out that not all attack based champions are "not worth investing  in". There are  other examples also

HP champs, facepalm, with redundant attack stats.