The answer to this question comes down to what type of pillow you have. If you use a latex or memory foam pillow, it's much less likely that dust mites will live there. The structure of the pillows makes it much harder for them to pass through or reside inside. With this being said, using a pillow protector will undoubtedly preserve the life span of these types of pillows.
For anyone who has opted for a hypoallergenic pillow filled with synthetic fillings, we suggest using a pillow protector. Their fillings make it much harder for dust mites to pass through than pillows with natural fillings. However, they are much less effective in keeping allergens at bay than latex or memory foam pillows.