I decided to give your claim a semi-scientific test (to the extent that is possible with one computer). So what I did was download CCleaner (portable), then use the windows on-board tools to clean temporary files. I then ran CCleaner to Analyze what it can find.
Interesting fact #1: It found a whole whopping 839MB to clean up - "Now that's a lot" one might think and I was suprised at first. I was expecting it to find something as surely no tool, downloaded or on-board is all encompasing (sp?) but I wasn't expecting almost an entire Gigabyte worth of data. But looking further into it, I found:
interesting fact #2: From these 839Mb of files 800Mb are allocated to "Utilities - NVIDIA Install files". Now, there's a reason I keep these specific install files on my Laptop and I wouldn't want those deleted at all. Why in gods name CCleaner decides that I am not going to need these files that are very important to me is beyond me. But hey, I can disable that. So we're now down to 39Mb of remaining files that CCleaner could clean that the windows on board tools could not.
Now you might want to call me wastefull, but let me point out: the last time I actually cleaned my harddrive was over a year ago. Way over a year ago... But 39Mb of storage space over a timeframe of, say, 1.5 years is
a) not worth anyones time
b) insubstantial enough to qualify *not* installing additional software on your PC that might introduce security flaws, adware or even malware.
c) potentially risking important data (see my specific example about NVIDIA install files that I actually need to keep.