Why would they use B-24's as "Flak Screens" to protect B-17's?

The B-17 had a higher service ceiling (~35k feet to the Liberator's ~28k) so they could fly higher. As demonstrated in this video flying high was good because it made the flak take longer to get to you, giving you more time to move out of its way. Now, the B-24's were also going to be trying to avoid being hit--they wouldn't take a hit to spare a B-17 from being hit. I am very confident that giving the -24's a disparaging nickname was a matter of intraservice rivalry rather than any kind of policy. (Edited to add: On a practical note, only those antiaircraft shells with an impact detonator would be "filtered out" anyway. Those set to explode at a preset altitude would explode near either the Liberators or Fortresses, but not be triggered by the Liberators on the way up to the Fortresses without a direct hit. The Germans lacked proximity fuses during the war.)

~~However, one of the primary virtues of the B-24 was that it was less expensive than the earlier B-17.~~ (Edited to correct myself, apparently the B-24 ran a bit more expensive on average than the B-17. The tradeoff was less complexity for increased range, bomb load, and speed--not cost) The B-17 was an incredibly robust design. Rather than being engineered just well enough to accomplish the job, the Flying Fortress was overengineered. This generated additional expense and complexity, but it resulted in a very survivable airplane that could absorb damage and keep flying. The B-24 had longer range, a larger bomb load, and was less complex, but it was also more fragile. Its fuel tanks were more exposed, the high-wing (rather than the midwing design of the Fortress) made belly landings more dangerous, and it was a more fragile design overall. Still, there was no way that the B-24 would be sacrificed needlessly. Even being less expensive than a B-17, the cost of a heavy bomber like the B-24 meant that it was a resource to be spent very carefully--as were the lives of its crew. It is for this reason that US bombing of Germany was halted in 1943--losses were unsustainable and you just couldn't afford to throw expensive bombers away.

As always, followup questions by OP and others are encouraged.

Edited to add: There were other nicknames given to planes during the war as an insult. Perhaps the most common was the "Jug" nickname given to the P-47. Originally it was given the name because it resembled the not very threatening profile of a milk jug. The P-47 was heavy, ugly, and huge compared to the nimble Mustang and the agile Spitfire. However, it later became a term of affection once the plane proved itself in combat--especially when it was able to absorb astonishing amounts of damage and bring its pilot home safely. Unlike the Merlin-powered Mustang and Spitfire, the Jug's huge radial engine could survive being hammered by enemy fire and continue to function--and the rest of the plane was just as robust. The 'jug' appellation eventually became a term of endearment rather than an insult.