what I saw as a bug was not the fact that grate don't count as support, but the fact that it is possible to build as if grates where counted as support, which got my dwarf gravely injured.
That the game allows designation/building of constructions does not mean it suggests the result is supported; that's something the game simply doesn't care about before the building is finished and physics kick in. Work safety is the overseer's obligation, and it's entirely possible that a building unsupported at the time it was designated would be supported by other, earlier-finished buildings by the time it got done. The game just doesn't second-guess your designations and is a tad optimistic in the assumption that you know what you're doing
I find that the lack of pre-checking, while demanding more attention, provides tangible benefits: unchecked building means it's easier to designate/construct with scaffoldings (auxiliary buildings like bridges to allow getting to the worksite); it also means you can willfully cause cave-ins through construction; and toggling the "allow cave-ins" option in the init files causes no difference in builder behaviour.
In a nutshell: i think an argument in favour of pre-checking building designations should be filed in the suggestions forum, not as a bug report.