My Paterson System-4 tanks don't leak either. There are a lot of older Paterson tanks (with screw tops, white plastic lids etc) which are so old now that the original soft plastic seals are either hardened or missing - perhaps it is those that people have problems with? Anyway, the 'tupperware lid' style tanks work fine for me.
Inversion agitation also seems to give better agitation, if there is enough air volume in the tank to let it work. That could be tricky if the developer is highly diluted and you need a large volume to hold enough active dev, but possibly in that case one would be (somewhat optimistically) trying a variant of stand-development and using minimal agitation.
Mind you, there is the current (Tupperware lid type) Paterson Super System 4 and the older (screw-on type) 'non-super' System 4.
IME, the older system leaks almost invariably, while the latest version does not, or only if you let too much pressure build up (easily released).
Tea Leaves are good for this sort of flow visualisation - I have used them many times at work to try and understand water flow behaviour.
If you think about it its not surprising that constant uniform rotation does not agitate the developer - there is almost no relative movement inside the tank between the surface layer of the film and the chemical.
What agitation is supposed to achieve is to remove the depleted surface layer of developer with fresh chemicals.
Whether you use a flick of the "Twizzle Stick" or use inversion, the effect is the same, lots of relative movement between the film and chemical - allowing fresh chemical to replace the stale layer at the film surface.
On the subject of Paterson Tanks leaking, I have just invested in some new Paterson Tanks and have been pleasantly surprised how leak resistant they have been.
However, I still wear Nitrile Rubber Gloves - a friend after many years suddenly developed an allergy to Metol, which is a common component in many B&W Developers.
He nearly had to give up Photography because of it