This is what the big commercial units use. The surplus means extra power that thermal blocks just can't match. Believe me, testing told the tale. Boilers made more authentic-tasting espressos. The heavier the filter holder, the better.
Temperature is crucial to brewing good espresso, and brewing occurs in the filter.
This thing is constantly cranked in and out of the machine, and it's banged around a lot. A lightweight filter holder can't hold up to that kind of wear and abuse. Eventually, it may not even fit securely into the machine any more. My six picks do a good job of frothing or they wouldn't be here. And capacity isn't such a huge deal either, since any pump machine can be refilled even when it's running.