Anyone who's ever tried to earn a living making music would know how hard it is to come up with something original and find a unique and pleasing voice on your instrument.
Whether you love or hate Malmsteen, he was the figurehead in starting the neo-classical guitar music of the 1980s and his feel and phrasing on the guitar sets him apart from many imitators. There are many faster shredder style guitarists but they sound lifeless in comparison to Malmsteen, with his organic tone and vibrato as well as his classical influence.
So while I agree he is easy to tire of because he hasnt changed much in his musical style and ideas, and he is a bit of an eccentric personality, I think he has contributed something to the world of music that is more than "fret wankery". His Live in Leningrad concert DVD is pretty amazing and probably his best period. I agree his latter albums are not very inspired.