We really shouldn't.
I'm not even sure where to start with this one. Look, terrorists are generally not stupid. They've known for decades not to use open phone/internet lines. Sure, there are exceptions - Lee Rigby's murderers used Facebook to chat about killing a soldier, as just one example. But, sticking with that example, Lee Rigby was murdered by two guys, Michael Adebolajo and Michael Adebowale, who were already known to the security services. Lee Rigby was killed in May 2013, Michael Adebolaje was on the MI5 radar from 2008.
Fact is, the intelligence services already knew about his killers and their links to terrorism well before the attack. This is something that comes up time and time again - when an attack happens, it then gets revealed that the perpetrator was already known to the intelligence services. From an article by Glenn Greenwald:
"For most major terror attacks, the perpetrators were either known to western security agencies or they had ample reason to watch them. All three perpetrators of the Charlie Hebdo massacre “were known to French authorities,” as was the thwarted train attacker in July and at least one of the Paris attackers."
So how exactly does the Snooper's Charter help us? Well, it doesn't, really, plus there are a whole bunch of negative effects. Essentially, the Snooper's Charter is a tool for mass surveillance of phone networks and the internet. There's other parts to it, like allowing the hacking of a suspect's phone or other electronic devices, but those bits aren't exactly giving new powers. The real meat lies in the mass surveillance part.
There's a whole bunch of reasons why mass surveillance is a bad thing for individuals and why the right to privacy is important, but rather than write them here, I'll link to this video (again, a Glenn Greenwald one). Actually, the full talk is good watching - if you want to watch it all, it starts here.
In short, the Snooper's Charter, or Investigatory Powers Bill, doesn't keep us safe - no safer than actual intelligence-led surveillance does, and probably makes us less safe. I mean, one of the problems with the NSA surveillance is that there is simply too much data to meaningfully process. Couple that with the way surveillance powers historically have been misused in the UK (eg, spying on parents to make sure they live in the right catchment area for schools - and that's not a lone example), there's really no way you can argue that the Paris attacks are a justification for more intrusive mass surveillance here in the UK.