I have never used them, they get in the way and look terrible.
Some people seem to think that they aid air flow, but as far as I can work out they are really just sh*t guards to keep the engine clean.
Again my engine bay has never got that bad. Although it is all painted and engine lacquered so it's a lot easier to clean.
This is just an opinion and I'm quite prepared to be wrong
If you do want them and your handy in the workshop look at this link
http://www.teglerizer.com/triumphstuff/spitenginep...
Jon