Everything in the store that costs RP is purely cosmetic, they don't give any sort of advantage to one person over another, so no, skins should not be available for free. If you want a skin, then dish out a little money to help support Riot. It's not much for a skin considering the value of the game your getting to play for free.
WoW has a subscription cost of $15 a month, yet they still have a micro-transaction store, for cosmetics and their prices are 2-3x the cost that Riots is (battle pets $10 : ward skins/260-520RP skins, mounts ranging from $25-$30 : ultimate skin) and yet LoL is still free to play, I think were getting off pretty easy. imo.
So if you want a skin (maybe even 2 pending on release date) throw down like $10. If you don't got the money/don't want to spend the money, then don't. Your not missing out on anything.
P.S. a recolor idea wouldn't be terrible, but I think if thzts the case then it should be at the price mentioned in another reply here, if a champ is 6300 ip, then the recolor should be something like 12,600 ip or even 18,900ip, that way there is more of an incentive to push towards buying RP for a legit skin, or having the option to pay 2x-3x the champ cost for a recolor skin, to help with ip inflation, for those with mass amounts of ip with nothing to do with it and for those who are unable to afford / choose not to purchase rp. I know SMITE does it, I believe once you've reached a certain amount of games played with a certain champion (or a certain amount of wins to gain "favor/rep" with a champion) you unlock the option to purchase a gold recolor of the champions for their version of IP