a solid ujoint is great for a race car that sees 100 miles a year but for a street car that sees thousands of miles needs to be greased periodically. the zerk makes that possible with ease. ive run the hollows on mine and never broke the cross shaft. i have seen lack of greease destroy rollers and caps though.

so i would think that if you were running a 11 or 12 second street car that sees track time, you need the greaseable fittings unless you want to pull the ujoints out every year to grease them