This is coming from someone who plays shadow very rarely (as it lacks most of the "fun" spells in Vanilla) so take what I write with a grain of salt.
Its usefulness depends a bit on how you want to play and what kind of stats do you get from your gear. Mind blast causes more pressure in relation to your other damage spells when you don't have a lot of spellpower. Although, as I see it the spell's real value is in being very good for baiting any response from the enemy (interrupts, ice blocks, reflectors, gouges/kidneys, etc.) as most of the interesting opponents won't allow you to freecast it.
Anyway, I'm unable to see being able to cast a mind blast one global earlier very groundbreaking so I wouldn't take it.
The alternatives are simply better to even really consider going for it; you'll want to have Healing Focus in pretty much any spec, extra armor is nice against geared physical damage classes which Kronos seems to have a plenty of, having a reduced mana cost on instant casts is great considering you'll cast a lot of instant spells and as a vanilla sp you'll generally struggle with mana sustain in longer fights.
In the specs you posted you also have one extra point in shadow focus (just put it into spirit tap) and if you're able to itemize hit, which you should do in order to avoid dispel resists, you can leave it empty altogether.
If you want to watch pvp movies, the only vanilla priest videos I've found relevant for today's standards are Zzi's movies (admittedly I don't watch a lot of sp videos).
Ultimately, if you want to be a dark lord of death and destruction just roll a warlock. It's superior and more rewarding to play in pretty much every regard, a vanilla shadow priest is more of a support caster in most situations. The only real upside to vanilla (shadow) priest is that it's simple to get into as you won't have very many offensive buttons to choose from and it's easy to take on unprepared opponents with.
Obviously there's some finesse to be had as shadow just as there's with every class/spec combination, and in my own opinion it's harder to be actually useful/play to the fullest in teamplay compared to vanilla disc (I'm biased as I mostly play disc, but shadow does have more limited mana, more cooldowns to play with and an extra gcd for casting holy spells which people seem to rarely give any merit to), but it can get pretty dull compared to playing an actual dps class.
