I agree with what you write in your post. To play the game successfully, you need to own your cards in the long run. It's possible to go up in the leagues solely with renting cards but you totally depend on the price swings and some seasons you might actually lose money rather than to win. At the moment cards are rather affordable and I think it's a good moment to buy them on the market.
I have picked this post on behalf of the @OurPick project and it will be highlighted in the next post! 
So true. The price swings can really hurt if you're renting a lot. Renting is a great solution for short-term needs. It's also a good fix to supplement your own collection. But if you think long-term, owning your cards is a better goal.