What I don't understand is, if they meet the DPS checks, what's the overall problem here?
Let's say SE does that calculation and determines the sum of the basic DPS four DPS and the tanks do in an encounter comes out to 100 DPS (just to use an easy number). They then take this number and cut it as mentioned, so the raid requires 85 group DPS minimum to clear.

It is safe to assume that the established theorycrafted rotations and playstyles will result in close to 100% of your Job's theoretical maximum DPS. Obviously in a raid environment, this is an unrealistic number, but we can still consider it the DPS ceiling. So let's visualize what we have so far:

Theoretical maximum DPS (approx. 100 for this encounter, and only approachable with theorycrafted rotations and playstyles)
P
P
P
<-----Passing zone
P
P
P
Minimum DPS required for encounter (85, in this case).
F
F
F
<------Failing zone
F
F
F

With everyone in your party using established rotations, your party's damage ceiling is virtually at the top of that chart. Those six Ps under it can be considered breathing room to allow for human error and mistakes.

However, if you (personally) are using a suboptimal rotation, your party's damage ceiling now goes down, say, to the first P. Now you only have five Ps of breathing room. But wait, everyone in your party read your guide and is using a suboptimal rotation. Now, your party's damage ceiling is only a P or two above the minimum required to pass the encounter. If someone dies, if someone lags, if someone has a brain fart, if someone fat fingers during a three-step combo, if anything happens, you go into the F zone.

At best (the first half of the preceding paragraph), the player is intentionally lowering their groups damage ceiling by using an "easy" rotation, therefore lowering the margin of error and placing more stress on the other party members. At worst (second half), everyone has the "I only need 85%" mentality and therefore places the group in a precarious situation.

The "I only need 85%" mindset only works if a minimal amount of people in the group has it. Can you truly believe such a mindset can then be called a good one?