I can't answer why anyone would take VL over RoS, but as Shadow you're being trained 99.9% of the time and taking San'layn with Shadow Mania helps keep yourself up quite a bit(if you're getting your VT's off), and even helps with passive insanity gain. Psychic Link just isn't that good even w/ Shadow Insight because you need to be prioritizing VT uptime more than anything else.
Going Shadowy insight is a viable option, however Mindblast damage is very low compared to VT/SWP damage and having VT/SWP on at least 2 targets 100% of the game is your absolute primary target for pressure and self heal even without shadow mania. Shadows survivability is very weak and both San'layn and Shadow Mania increase the self healing. SM also provides a steady insanity income and SL increases your VT damage. Psychic Link is an option for RBGs but i think it needs a buff. If you take Psychic Link you should also take Shadowy Insight and Fortress of the Mind, but i think it is only viable for RBG.
Im assuming they are expecting to not get focused, which means they have a grace period of 8 seconds where their haste can go sky high. But it seems wrong, since pvp is all about nuking out that last bit health before the healer gets anything done.