Beside making it impossible to tune the engine well it can cause serious problems. Air in the line causes the engine to run lean. If you're lucky it's bad enough that it just stalls and stops; a simple dead stick (if it's on take off it's not so simple). If not it can cause an overly lean run that will overheat and swell the components and scour the cylinder and either fatally seize it up or kill the compression and ruin it.