Although being able to see an ally's number of rerolls would be somewhat beneficial, it's unnecessary since you can already say you have rerolls in chat.
As for Ranked ARAM, it wouldn't be a good idea since ARAM isn't truly "All Random." Quite a few players tend to have ARAM accounts which only contain certain Champions based on an "OP-based" tier list. Even if all the Champions were unlocked, Ranked ARAM wouldn't be a fair demonstration of skill that Ranked mode normally allows in Summoner's Rift. More than likely, many players may receive unfamiliar champions. Thus, they may not be at optimal performance and will be already at a disadvantage at the start of the game. When faced in this situation and limited rerolls, players may be more tempted to dodge at the expense of LP rather than suffer a loss which may cause them to lose more. This factor leaves the Player a victim of RNG and results in a lose-lose situation; which in my opinion isn't fun or fair.
TLDR; RNG shouldn't really be a deciding factor in winning games, especially in a mode as important as Ranked.