Dom has this slightly wrong as the PCV valve is attached to all cylinder inlets, not just the front 3 cylinders.
the PCV valve allows oil vapour to suckked into the engine only when the inlet manifold is under vacuum, e.g. at idle, at cruise, at low load on the engine.
it is perfectly acceptable to do this.
Is also acceptable to vent the crank case pressure and oil vapour directly through a filter into the engine bay, not through a catch can.
but it is more environmentally friendly and cleaner to burn the oil vapour in the engine via the PCV valve system and to collect the remaining oil vapour in a catch can and if an oil drain/return pipe is fitted then the condesnsed oil in the catch can will flow directly back to the sump so you do not loose any of it from the engine.
the pipe from the inlet pipework to the oil catch can, just aids in the collection of the oil vapour, as a small vacuum is created in the inlet pipe work and helps to suck the vapour into the catch can. but the crank case pressure is pushing the vapour into the catch can anyway, so venting the catch can with a filter if perfect OK as the oil vapour has been caught in the can anyway.