"Some Engineers" according to what the wall is used for, if its the inner skin of a cavity wall then is should be made good with a sand and cement this would be in the specification, in most case's the chase is left open for the spreads to fill with plaster, that is why "Some Engineers" are concerned, the inner leaf of a cavity wall can be 3.5N, 7N, or even up to 22N in a 3.5N block a chase filled with Bonding would be close the crushing strength of the block, but the Engineer can not rely on the site management or spreads to follow the specification, the days of the "Resident Engineer" have long gone and was the only way of ensuring the specification was followed, why Structural Engineers no longer have to spend six months on site before becoming qualified has baffled me for some time, this also applies to Architects, neither now days have any experience of what used to be called Buildabilty.