The problem is, "necessity" or "usefulness" are used as substitutes for legality.
"Well, it's not, technically, one hundred percent legal, but look at how well it works in fighting [terrorism|pedophilia|some other big cause that gets people all worked up without actually threatening anyone in any statistically meaningful way]".
Uh, paedophillia is statistically significant. The wider issue of child abuse (physical, emotional, sexual abuse or neglect) means that about 5 children a day die in the US from abuse-related causes.
Numbers are difficult, but over 3 million reports (for over 6 million children) are made every year in the US, and about 9% will be for sexual abuse. (And about 90% of those will be where a child knows the offender in some way.)
Very many people are harmed, sometimes severely, by paedophiles.
Please don't ever place paedophilia in the same category as 'nonsense boogeymen' like terrorism.
We could castrate everyone even accused of pedophilia and that would probably discourage some amount of child sexual abuse, but clearly that wouldn't be right. Similarly, we could forcibly commit everyone with a history of mental illness in mental institutions and maybe that would've prevented Sandy Hook, Aurora, and most recently the Santa Monica shooting. Yet, that wouldn't be right.
In short, the ends don't justify the means. Usurping constitutional rights because it would make it more convenient to fight terrorism is the worst possible path we could take in dealing with the threat of terrorism.
They weren't saying that they were trying to convince Congress that it was "legal or necessary", which is what you seem to be complaining about.