No, you do take imp. Mind blast for PvE because once you get meditation there is no reason to put more points into any other tree, which leaves you with more points to drop in shadow, meaning that those 4 points in imp. mindblast are super awesome because it allows you to chain mindflayx2 during mindblast cooldowns. There's no reason to put yourself at a potential disadvantage for DPS by having your hardest hitting attack be on a longer cooldown, especially when you can spec it to coincide precisely with your main nuke. not putting any points into it means that to max your dps you have to clip your third mindflay's last tick.
This is the standard PvE shadowpriest build during this patch: http://db.vanillagaming.org/?talent#bxx0sZZxGxpcxRt
The only way any argument could be made against it is if you REALLY want to put 3 points into mental agility, but since it only affects SW:P (and DP if you're an UD), it's 4 talent points to save 29 mana over 30 seconds, which just isn't worth it imo.