The Outer Marker or DME Altimeter check is simply to make sure your Altimeter reads correctly. If you had a 220' minima and the Altimeter over read by 220' you would get a surprise on breaking visual at minima. The unexplained error comes from the question "am I on Glide Slope, above or below it? If you are 1/2 scale above you would be at a greater height and that would be an explained error. Now, you only have to quickly mentally work out "I am 0.0X degrees high at X distance, using the 1 in 60 rule, that makes me XX feet high. The check height + XX is what I had on the altimeter, therefore my altimeter is correct" Crunch, o s**t wrong calculation.
When you consider that in ISA the first millibar drop occurs at 16' and the difference between 100 000' and 200 000' is also 1 millibar, then we can see that the altimeter is a bit of a rubber band when it comes to measuring height. Normally that does not matter because the aircraft next to you is also measuring with the same rubber band. However when you approach the Ground it is much more 'fixed' and therefore much more caution is required. The airlines use Radar Altimeter height and even that varies with the aircraft's pitch angle.
As far as 1/2 scale or full scale goes on Glide slope or Localizer, if you are 1/2 scale you can still 'see' where you are. Once you have full scale, how would you know whether you are 3 degrees off or 30?