A bottomless portafilter can help dial in the all the elements of pulling a great shot (grind, tamp, extraction time) by giving immediate feedback to the barista. Channeling of water caused by poor dosing and tamping can easily be spotted by little "spouts" erupting from the basket. Mottling or Tiger-Striping can easily be observed indicating a proper extraction.

Also, look at the Rancilio Factory Cut Bottomless Portafilter with Triple basket.  The Rancilio has an ergonomic handle and will fit the same machines as this portafilter.  If you have any questions, as always, we are glad to take you call or email.