Why do concrete blocks [whatever you call them in america] have hollow channels in them?

A couple reasons. Firstly, the extra mass is not required structurally. If it was you would probably pour your wall instead of using blocks. Secondly, the hollows are there as insulation. Filled with insulation or unfilled they still have more R value than concrete. Thirdly, weight. A thick wall is structurally more sound than a thin one of the same mass.

unfortunately that is incorrect. They were designed to be filled. Sort of an early version of that styrofoam insulated form work that you stack and then fill. The problem with leaving them empty is as numerous homeowners can attest to, leaking through them. Filled, no such problem arises [through the blocks at least]. I've also notcied that you get muppets thinking you can take them as high a you like and leave them hollow. The hollows allow reinforcing to be used, provided they are also filled with concrete. Now you may ask why wouldn't just pour? Simple, no need for forms [which in some situations can be expensive and a right bugger to use]. I've heard so many people here in canada say the hollows allow for greater R value, then they turn around and wonder why the bloody basement floods in the spring.

The leaking problem has been addressed by waterproofing the outside of the walls with a membrane which is applied with primer,mastic and rolled on( building codes for commercial/government buildings require this)

Typically, the footer is cuffed slightly to allow what is called a 'way' to allow water to run the length of it,downward to an outlet.

Water penetration isn't all that prevalent in cinder walls, just the cases you've probably heard of(how many have complained that their walls weren't leaking... )

I grew up with cinder foundation,living on a drained lake basin in Cincinnati,Ohio,yes we had leaks,yes we had to excavate to stop this, yes it did stop, but this is just the way it is.

With time and experience, the building trades have all but perfected this application, not just from commercial,but throughout all residential.

I have seen both arguments with filling and not, if we wanted to fill we would use finish block(solid) throughout, or a course of thermite(sp)(solid) and a few open ended then fill with concrete for attachment of plates to the foundation.

Docksplank(interlocking cinder block w/hollow core)is also used for horizontal applications(ie,interior garage ceilings) and while being hollow(and some with earth over top) injections with insulation/waterproofing materials have been quite adequate.This application would not allow for filling with concrete as the weight would pose a problem.

Cost is he real issue I see here. Are we(Americans) about cost vs. quality, sure, who isn't?

If I have to fix something which might not be up to MY personal standards in a home I bought,so be it,BUT... If I am building for my own use,weighing cost and quality are not the same.

Just my opinion,

Ken