In the Dungeon Master's Guide for the Use Magic Device feature of a rogue it says on page 141 that "your proficiency bonus does apply" for a magical staff. Does this count as being proficient in the item? If so could a rogue wear magical platemail or use a longbow with no downside?
Spells [...] A magic item, such as certain staffs, may require you to use your own spellcasting ability when you cast a spell from the item. If you have more than one spellcasting ability, you choose which one to use with the item. If you don't have a spellcasting ability-perhaps you're a rogue with the Use Magic Device feature- your spellcasting ability modifier is +0 for the item, and your proficiency bonus does apply. (DMG 141)
I have quoted the whole paragraph and the title to clear up the misunderstanding. The half-sentence you quote refers only to casting spells from a magic item and the use of a spellcasting ability when you do so. It does not mean that you are considered proficient with all uses of the item, just spellcasting.
USE MAGIC DEVICE
By 13th level, you have learned enough about the workings of magic that you can improvise the use of items even when they are not intended for you. You ignore all class, race, and level requirements on the use of magic items. [PHB, pg 97]
You do not gain proficiency, only the ability to use.