They can hurt cooling and my experience has been they are not needed. I've built motors with and without them over the past 8 years or so and my opinion is they are a waste of time and can create alot more problems than they fix.
ooooh yeah. that is righ, i forgot about that. he is right about the cooling. dont they drill holes for better cooling? IF you decide too use a block guard its not really necessay untill 300hp i believe. im not using one in my ls thatll be fully built. it wont be powerful wnough. ive rebuilt an a6 and KJ has aparently built a few more motors than me, so id listen too him. hes got more experience.