I agree Symantec has made huge improvements in the way NIS uses resources, and so that is probably not a valid reason to avoid it any longer. That said, in my opinion, ALL suites hog resources, typically because they contain features most users just don't need, or want.
I have two reasons I'm not crazy about NIS in particular - well, four really, but I'll get back to them later.
1. I don't like how it comes bundled on new systems and then marketed in such a way that users, now used to the UI, are intimidated to remove it or try something else when the trial period expires, for fear something will break and the badguys will take over.
2. It rarely uninstalls cleanly, often leaving not only orphaned files and folders, but (and this is what irritates me) services and applets that continue to load during start up. It is typically necessary to use special removal tools to rid the drive, and Registry of all traces.
Now back to the other 2 reasons - which are not exclusive to just NIS, but to most suites from the big players.
3. They costs money - not only to buy initially, but there are recurring costs too. When there are many, fully capable, less intrusive "free" alternatives out there, I just don't see why home (non-commercial) users should have to pay.
4. Way back many years ago when Microsoft really was trying to rule the world, MS tried to put, among other things, an anti-virus applet into Windows because they foresaw virus activity was increasing and more and more badguys were discovering how profitable, and easy to exploit, this new thing called the World Wide Web was. But Norton/Symantec, McAfee, CA, and a few other early AV makers cried and whined to Congress and the EU that Microsoft was trying to monopolize the world and push them out of business. They testified it was their job to rid the world of viruses, worms, etc. And Congress and the EU, already hounding MS for their strong-armed marketing techniques, agreed with the AV community.
We see how well that worked out!
They failed miserably! Malware has mushroom insidiously, not only in quantity, but in maliciousness and in methods of delivery. And in the meantime, the biased IT media and Microsoft/Bill Gates bashers have relentlessly blamed Microsoft, and not the badguys, or the politicians who failed to fund law enforcement of even the existing rules against malware and software piracy (a major source of malware).
I ask, "what incentive does Norton/Symantec, McAfee, CA, Trend Micro, etc. have to rid the world of malware?" They don't have any! If they stopped malware at the source, that will put them out of business and they don't want that.
Microsoft, on the other hand, has huge incentives to rid the world of malware because they keep getting blamed for the security situation we are in today - even though it is the badguys (to include corrupt ISPs and 3rd world government leaders) who put us here.
So now, there are several excellent anti-malware products, to include Microsoft Security Essentials (which I use on all my systems) that are totally free, not huge resource hogs, and do not contain all sorts of unnecessary features. Most importantly, these programs are just as capable of protecting us from the badguys. They cost nothing initially, there are no recurring renewal fees, and there is plenty of free technical support available. For those reasons, I cannot justify paying for, or recommending a product that offers no real advantage over something we can get for free.