Heavy and impractical, definately. But they were issued to Seals for a short time. Maybe grappling hook wasn't the best term... an 'ancor' probably would have been better.
More informatin about it can be found here:
The Buck-184 Buckmaster - The peak of survival knives!
Some highlight from the article:
"Regarding the grappling hooks, these are described it the patent which
CAN BE SEEN HERE. In brief, the patent discusses the attachment of the grapples to the quillon with sufficient strength to support a load of 600 pounds and reference is made to a casting line. With this said, it is generally accepted that using the knife as a grappling hook, which implies the act of tying a rope to the pommel and throwing the knife, could be very hazardous! In fact, the last of the BUCKMASTER knives came with small flyers saying that using the knife as a grappling hook would void the warranty. A better use of these hooks are as anchor pins where knife can be wedged in some rocks or the crotch of a tree and used with a securing line off the lanyard loop attached to the hollow handle cap/pommel."
"As for the knife's production, the Buckmaster was tested by and issued to the Navy SEALs for a short time. I have read that the knurled metal handle did a number on hands while chopping; it didn't do too well in salt water tests; and it was very heavy."