The biggest problem with Phasic Cloaking is it's cost, at Tier 4. If it was Tier 1, it would be fine since, on larger(multi-star) maps at least, neutrals would still be around and there would still be places to search and free asteroids to capture. On Tier 4, it's useless.
So the Jikara Navigator costs 25 cash, 25 metal and 1 supply point more than the other scouts, and doesn't have any special attack like the Arcova's time bombs and the Seeker's suicide ram. Instead it can capture neutral extractors, it has slightly more firepower, and it has a useless and overpriced ability.
Buildings don't have Phasic Cloaking, only scouts do. Thats Phasic Barrier or something which you are thinking of. I guess agaisnt the AI that might be usefull, but agaisnt a player he's just going to sit there and blow it away when it comes back. As well, if it doesn't auto-cast it to do that, then I'm having to sit there and watch it. One of the great things about scouting is that after its done you can just mouse over the planet and see the report. You don't have to sit there constantly watching it... It seems like it'd be better just to let my scout out run all defenses, than actually spend time/resources researching Phasic Cloaking.
The Vasari Empire tech tree is filled with weird stuff. Raider Xenophobia, the Volcanic population upgrades, Phasic Barrier, Phasic Cloaking, the Nano Weapon Jammer, and the Jammer's upgrade tech are all of questionable usefulness.
As for letting the scout outrun the defenses, Gravity Tolerance is better for that. At least it affects all ships, but it's also kind of overpriced at Tier 5 when you're getting Phase Stabilizers.