According to Duke Le Jeune, a respected designer of bass cabinets, cone rock only occurs if the port is directly behind, but off centre, to the driver. In one of the threads in Talkbass he goes into detail about he got, and solved the problem of cone rock, with the help of Eminence's technical department.
This is opinion. Corner ports are a type of slot port and they break several of the conventions. Firstly as @Phil Starrhas already mentioned, @steviehas demonstrated to me and @Phil Starrthat a single large round port is better than several small ones*. There is no good reason to suspect that this is not true of multiple corner ports. So why do people use slot or triangular/corner ports. Firstly they act as bracing and secondly they negate the need for an expensive moulded port**. Remember that the wood that creates the slot or corner port is effectively an offcut of the main cabinet panels or a much cheaper material.
*This was why the Mk3 version of the BC112 cabinet had one large port instead of four on the MK1.
** A part costing £1 at manufacture will probably end up £3-4 on retail price.