I started collecting playable trading cards when I was about 12 years old, and I began playing Middle Earth about a year later. I have always used these thin covers to protect the cards that I play with. After all these years (and I'm not admitting to how many) the cards I started with are still in excellent condition.
The plastic is thin enough to allow basic shuffling prior to and during the game.
The benefit of the plastic cover is that it keeps the grease and acid which naturally occur on the hands from getting onto the cards, and when you have finally gathered together your perfect playing deck the last thing you need is one or more of the vital cards starting to fall to pieces.
Plastic covers also mean that, should you want to sell or swap one of your playable deck cards, you should be able to use the condition of the card as a bonus for extra points against the card(s) of your choice.
If you are planning to collect the cards, rather than play them, then I would recommend that you invest in some good quality card display pages, such as
20 x A4 MAX 9 POCKET PAGES FOR POKEMON YUGIOH AND MTG CARDS, and a good binder (3 or 4 ring to hold the card display pages straight).
These are superb for anyone who plays collectable card games and doesn't want their cards to get mucky.