I have one of these doofers, as discussed in this thread (and a tale of woe from one owner on this thread). In a nutshell, there is a (however slight) risk that a failed harmonic balancer can knacker the timing belt and cause some pretty horrible damage to the engine. TitaniumDave on NSXPrime came up with a relatively inexpensive guard (here) to prevent an errant HB from throwing itself at the timing belt, and has been selling them in the US for a while.
I bought this one as part of the UK Group Buy but never got round to fitting it, and Kaz replaced the crank pulley as part of the TB/WP service last year.
The cost to buy on in USD would work out about £50, plus P&P and the potential of duty etc. Or if you want one, make me an offer
cheers
Ewan