We won't ever have an answer to that question, but the same engineer or group of engineers who chose a sealed bearing also put them into the manual gearboxes used in the models equipped with the M96 engine.

Likely it boils down to what is considered an acceptable lifespan for the bearing and acceptable failure rate. With a powertrain warranty of 5 years or 60,000 miles, it was likely figured these cars would be out of warranty before any problem occurred and they were most concerned with low cost of ownership and most buyers trading in on a new Porsche every 3-5 years. Manufacturers rate seals, greases, and other specifications for their parts, like bearings, with an MTBF, or mean time between failure.

On paper, these bearings probably should have never failed, but we all know that in the real world, components can respond differently than in simulation.
Specifically on the seals, they are rated only for 250F degrees for short periods of time, and it's common for these engines to see that temperature and higher.

Likewise, the 52100 steel races also aren't rated for those sustained temperatures. Seals get hard and brittle and the synthetic grease packed into the bearings, that was rated for higher temperatures and longer life, gets washed out. Long drain intervals recommended by Porsche results in the oil sheering out of grade and with reduced anti-wear properties. As you can see, it's a slippery slope and not one single thing can be blamed as the smoking gun with IMS failures.
The argument has been made and the failure statistics from the Eisen class action lawsuit support that if Porsche had used open bearings, we likely would not see failures of the early dual row or later 2006-2008 bearings due to their low 1% failure rate, compared to the 8% of the single row bearing used from 2000-2005.

 Load capacity certainly has something to do with it, but the use of sealed bearing on the higher capacity bearings certainly led to the eventual degradation of the high load capacity bearings regardless.
If oiling was the only cause of failures, as some companies argue, the early dual row and late higher capacity single row bearings would have the same failure rate as the lesser single row bearing. Forced oiling of a ball or roller bearing in this application is not required - the M96 engine is wet sump. This means the IMS is submerged in engine oil. Secondly, the inside of an engine is a vortex of oil mist during operation. An open ball bearing only needs about 4cc of oil per minute for proper cooling and lubrication.
Ball bearings and even roller bearings have been used by many manufacturers for decades in engines and transmission, but without the inclusion of a grease seal.

With this in mind, use of an open bearing, along with increasing load capacity, is one of the fixes to the IMS problem. Later 2006-2008 cars should have the grease seal removed as early in the life of the engine to prevent premature wear and failure, coupled with shorter oil change intervals and higher quality 5w40 engine oils. Those considering an open (unsealed) roller bearing, remember that roller bearings may have higher static load, but can only handle a fraction of the thrust load the IMS bearing sees, so service intervals should be shortened over that of a similar rated ball bearing. Beware false claims of load capacity and lifetime service intervals on these bearings. No roller or ball bearing is a permanent fix, even with grease seal removed - the IMS Solution which is a plain bearing is the only true permanent solution.
So once again, we are to the question as to why Porsche put a grease seal...

We don't know, but I'm positive that if there had not been one and if a dual row bearing had been used in all M96 and M97 models, you likely would not be reading a post about grease seals or have concerns about IMS bearing failures.

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