They are sometimes necessary because at the position of the T where the open vent comes off there is a slight negative pressure which prevents the air in the system from escaping at this point while the pump is running. This can cause a build up of air which eventually leeds to steam. The old way of dealing with this was quite effective, it was to put a T one size larger than the pipe size at this point, i.e. a 28mm T for 22mm pipes. The expansion so caused allows the air to escape during normal operation. There is no benefit to leaving the air in your system so you can blead it through a rad vent that I can think of. You always get gas created anyway, you don't need your radiators to be telling you that. Let it go free!