In watching a lot of scrimmages lately, it seems like getting the Blue Buff is a more important objective then getting the Red Buff, and at certain points it is more important than getting second tier towers. Why is this?
It's definitely more important than red buff. It allows the ap mid to spam abilities all day, whereas red adds a slow and some minor damage. I wouldn't say that it's more important than second tier towers, and I would say that there might be something you're missing in games where it seems to be that way. Perhaps they weren't strong enough to take down the second tower safely, but they knew they could take the blue. Or they knew that dragon was coming up soon, and they had time to take blue and get drag (or go back and get drag) but not get the tower in that same time frame. Or perhaps the ap was oom, and they needed the poke to secure the tower kill.
Blue is 100% vital in the start and middle parts of the game. The fatal flaw of casters (Bursty by nature) is that they lack sustain because of mana and cooldowns. Because blue provides relief to both of those, not only does it help them win their lane, but it helps them get more CS. This is essential to keeping your AP carry nice and fed, which is needed to nuke a person down in a single combo.
Why is it more important to red though? Simply because of the passive nature of bot lane. Yes, red buff makes you win fights as an ad carry, but you don't fight much at bot lane. So its not as important. PLus, red buff helps the jungler kill things faster, so he usually picks it up until team fights start.
The only exception is with M5's gameplay at IEM Kiev. They used GP to pick up red buff at the start, blue pill home, and teleport top (Their red btw). This gave his an extreme advantage at top, which is just like mid.
Maybe some teams will start giving their top red more often, and see how much it helps them.
It is team comp dependent, if your lanes are made up of manaless champs with already low cooldowns, blue buff doesn't really add much to you. If the enemy team is similarly picked that way, again blue buff doesn't add much. For instance if your lanes were: Kennen vs Mordekaiser mid, Renekton vs Akali top, and Lee Sin vs Shyvana jungle, then none of those champions benefit greatly from blue buff. None of them are mana based, only some are energy based, but not likely to run out of energy easily, and their main harass have pretty low cooldowns. In this case, red buff is much more valuable than blue buff.
However if you want to dominate mid lane - which is an important lane because mid tower gives you the most map control over any other lane's tower, then to win mid lane where the AP Caster tend to be, you want to have blue buff, as it gives mana regen and 20% cooldown reduction. Blue buff allows the holder to spam abilities off cooldown which means they can freely trade/harass their opponent with abilities. For AP Casters, their abilities do not even trigger minion aggro, which means they can do it without any trade back except by the enemy champion. With the cooldowns, assuming similar levels of play, this allows them to win any exchange of damage and ultimately win their lane.
Red buff allows the holder to slow and apply a DOT each hit. This allows them to win their trades if in a lane with auto-attack heavy champs. The slow the red buff gives also makes it ideal for a ganking champion, typically the jungler, as they can potentially keep catching up to the target and reapply the slow from red buff on them simply by right-clicking them.