Riot doesn't endorse smurfing but they do give a reason to create ARAM accounts

WookieeCookie: We don’t endorse smurfing

Ghostcrawler: Taken on the whole, I’m not a fan of smurfing and worry the potential harm to other players outweighs the benefits to those players with unmet needs.

Still, ARAM uses the player available champion pool rather than the entire champion pool. The problem with ARAM accounts is that they have a pre-game advantage over accounts that already have all champions (maxed account). I don't think that a pruchase should have a permanent negative impact on your performance in any game mode, whether it's ranked of 'just a fun-mode'.

The only reason not to make it use the entire champion pool is because some people think it would do more harm than good. Let's put all the pros and cons in a list.

pros

1. Champion select is fair for all. No advantage based on the champions your account has. (Seriously, a maxed account should not have a disadvantage)
2. You can trade champions with any teammate rather than only when you have mutually available champs.
3. The reroll mechanism can be simplified (1 reroll per game played)
4. You can buy a champion that you would like to play in Summoner's Rift and not feel bad about it because he's weak in ARAM. For example you want to play volibear jungle, but it would mean buying a bad champ for ARAM.
5. People won't have to expect the opponent's team to be stacked with at least 2 OP champions. Games will be more random as there will be champions that you currently rarely see because they reroll them or dodge queue if they get them.
6. New players can choose to play ARAM if they want to learn new champions in a quicker environment and not just the free week champs. Playing a high variety of champions is said to be the best way to learn the game. (I know that some people don't like that, but there is no game mode that appeals to everyone)
7. When a new player likes a champ that they just played in ARAM and don't own yet, they might buy them instantly.

cons

1. New players who don't like to play random champions that they don't own will be deterred from ARAM.
2. It will be harder to get hextech crafting keys and chests in ARAM as you need to own the champ (tbh I think it's a stupid rule that you have to own the champ. It disencourages people to try out free week champs and thus slows down the speed at which you learn the game).
3. People might dodge more (unless point 5 of pros has a big impact). Therefore I think it's important that with this update, Riot should do something about queue dodging in ARAM which is already problematic (suggestions are 1. give people who dodge the same champion next time they queue for ARAM; 2. Give people who dodge a low priority queue or a 1 minute in queue penalty so that they can't actually completely circumvent the punishment by logging in on another account or with another activity).

To sum it up, the biggest con seems to be for players who only like to play a specific set of champions. However, in my humble opinion, ARAM is for those who want to play anything. Perhaps there are a few exceptions in champions that you like, but there's still the reroll AND the option to trade with teammates for that. When I play ARAM, I want to play a champion that I'm not entirely comfortable with (not one of my go-to ranked picks) against other people who are playing a similar not-so-comfortable champion. Yesterday I played Orianna for the first time in over a year and I expected to be absolutely horrible with her and even though I still lost, I had a blast that game because I was able to pull off a few good ults and felt like I was better than expected.