I carry the extended spare without the plastic filler, which is useless in my opinion.
I'd need to check this with the Shield, but with SOME pistols, if reloading from a Slide Locked Open (emergency reload) the sleeve prevents Over-Insertion of the mag into the pistol.
With a closed slide, it's not an issue, but with an open slide (again, with
some brands) with extended mags, if slammed home, they can over-insert, bending/breaking the ejector and (possibly) damaging the top of the mag.
[Edit to add]
I just checked this and YES... With the sleeve removed and the slide locked open, it IS possible to over-insert the 8 round mag into the shield.
The top of the mag does make contact with the ejector and Could bend/damage it, if the mag were slammed into the mag well.
Naturally... under normal, UNstessfull instances, one wouldn't 'slam the mag home', but if being carried as a spare, or even during competition, it's easy to apply more force than 'normal'.