Leaving the HD 58x driver exposed like that invites in the dust & may probably damage it, so it's not recommended.
If you want to have them without the stock foam you may glue some other specific foams using double-sided tape according to mod guides on the diyaudioheaven for example. Such foam will alter the sound frequency response a bit (maybe even in desirable way) depending on which one you use. More detail in the guide.
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