very debateable!
I put one on my Bandit 10 years ago from new. My Fazer 11 year old has none. If you look at the vital areas like rear shock, underbelly, linkages ect ect.....there is not much difference between the two bikes as regards to corrosion after that length of time. The mudflap that is over my Fazers rear shock and swing arm seem to be doing an equally good job as my hugger on my Bandit in protecting anything in and around that area.
If I had a choice today to put on a £80 hugger or instead get (a) hagon fork springs £80, or (b) Engine bars £86, or (c) Front Braided hoses £88.....I would go for something to improve the bikes safety/handling first and the hugger would be my last thing on the list.
If anything, its the front that needs protecting. You never really hear of a rear shock corroded away to nothing on this forum, but you regularly hear guys in trouble with corroded front pipes. So I'd spend £20 on a extenda fender for the front mudguard and I could think of a hundred things that I'd do with the extra £60 like go treat myself to a brand new pair of gloves or a fresh set of rain pants ect ect which both are getting long in the tooth now and in which both would improve my riding comfort in these still cold wet mornings.