Why? :dunno Because they've never let me down I guess and they're readily available. Although I wouldn't by Denso etc from a shop if they had no NGK's
I'll probably put some iridiums in the gixxer next change. I never ran the iridiums in the TL because the manual called for a dual electrode plug that leaves the spark unshielded to the piston top. But the GSXR calls for el cheapo single ground strap plugs.
I use Irridiums in everything but the TLS. I used EK's in the TLS for ages and then tried a pair of Brisks (still in it). Couldn't really tell a difference with the brisks, but they have been in there for probably 30k miles so that's something I guess:O