of course there are legitimate uses for shareaza but with all the spammers out there you really need a
good security filter to weed out all the illegal perverted stuff. Like any internet protocol - http(s), ftp, etc - the Gnutella, G2, and ed2k protocols can also be used for good or for evil. The difference between shareaza and the world wide web (by which I mean http(s)) is shareaza's decentralized nature - one file usually comes from many sources - while on the internet one website comes from one place which is quickly shut down when discovered. Shareaza's strength which allows for fast downloads on residential internet connections is also it's weakness in that it makes it very hard for authorities to remove illegal stuff(although some would also view this as a strength - ie. fighting government censorship in places like syria , iran, and china).
At the end of the day, shareaza is a tool and you need to use in a way that is in compliance with the laws of your countryYou can find magnet links to all kinds of freeware and open source software on
freebase.be. Shareaza is also great for downloading one file over http from several mirrors at once(works on sourceforge, filehippo, linux ISO mirrors if BT is blocked by your ISP)