Outer Vinyl Sleeves, is it a good idea?

Depends how much ring wear on album covers bothers you, I suppose. I try to avoid ring wear so I use outer sleeves. 

I ditch mine immediately. PIA when taking records out of rack. In fact, I guess it was record sleeves that were the tangential inspiration for me to build my own record storage racks of the records are all front facing.

I use those really clear outer covers. Makes the album cover look almost new. No mold. Record store front-facing storage so much nicer than trying to read spines. 

I recently put my entire music collection into Discogs.  I decided on a guideline that any LP worth over $20 gets a protective outer sleeve.

I use them. Protects against ringwear. Never any mold issues. You can get sleeves that are crystal clear. Sleeve City is one company that sells them.