It's obnoxious, breaks entirely too many mods' multiblock configurations, and was entirely a childish, Greg-esque play to get mods to "respect isAir".

It's an unnecessary feature, and has caused pretty much the entire population of my server to ask for Railcraft to be removed.

1. isAir is a Mojang feature. Actually used and created by Mojang, your mod should accept it or face being incompatible with Mojang blocks.
2. It can be disabled simply.

Then please make it 'disabled' by default. The false bug reports are as bad as MCPC+ bug reports.

Not to mention, it prevents blocks from being placed, and causes unnecessary issue reports and bug reports with other mod devs.