Since it's inception at the start of season 3 the LCS has required teams to list at minimum two substitute players, and Riot currently requires a sub be brought to the World Championships with teams. Subs have seen play numerous times in the two years of LCS. From GGU's multiple sub attempts before settling on Jintae to replace Shiphtur in the inaugural spring split, to SK being forced to use Gilius to replace Svenskeren for their first three matches of the season 4 World Championships, substitutes see play quite frequently in the LCS. Despite the dozens of substitutions required in only two years of the LCS only one team has brought a sub into their gaming house and trained with them as an actual part of their roster.
Most teams in the LCS have a very careless attitude when it comes to substitute players. They treat subs as a check box on the LCS entry forms at worst, and as RoI (return on investment) streambots for their sponsors at best. TSM listed popular streamer Nightblue3 as a substitute player while he posted their sponsor's ads on his stream, but both parties admit he was never going to play in the LCS for TSM and when TSM made a change in the jungle they imported former Copenhagen Wolves jungler Amazing rather than pull up a listed substitute. Speaking of CW, they were last split forced to forfeit a LCS match when mid laner CowTard became ill and CW failed to get the paperwork for their substitute ready in time. When it came time for CW to make roster changes they again ignored their listed substitute Mazzerin, who unlike most had actually made it in for a game, and brought in a new player SorenXD as their new mid laner. When Gambit had visa issues that prevented most of the roster from attending the LCS road trip in England they scouted 4 new players to use as substitutes instead of pulling up either of their listed subs. Examples like this can be found all over the LCS where listed substitutes being used, much less becoming a starter, are an oddity rather than the norm.
Contrast that to other regions where deep rosters are used quite often. Samsung White's Looper is a well known examples to western players as he made his debut with the team at the Season 3 World Championships. LPL's OMG had trained enough with both their support players Dada777 and Cloud to allow them to switch between the players going from group stage to bracket stage during the World Championship and actually see improvement. There's also Chawy for TPA who played multiple games with the team during the GPL season and was reportedly on hand for the Season 4 Championships though not used.
Despite it being quite a common practice in the other World Championship regions the LCS regions have only had 1 single team really attempt the deep roster. That team was Evil Geniuses (EG) back when it was a European team. They brought in Shacker to train with the team and rotate with Snoopeh. The results were a clear success for the team. Snoopeh was fierce in pursuing his starting position again and played some of the best games seen from him in season 3, while Shacker also drove the team forward. Shacker's addition to the team in the summer split of EU LCS helped EG secure a spot in the 2nd-5th tie breaker matches. EG was able to go 2-1 taking 3rd for the regular season. Though EG did fall in playoffs to Gambit landing 4th place just outside a ticket to worlds the success of the experiment is still plain to see in their results.
Though the western scene has for the most part chosen to ignore deep rosters the benefits are plain to see in examples like OMG, and EG. A deep roster of live in substitutes gives teams insurance against whatever random illness or unfortunate life events might sideline a player, with live in subs a game is not simply a write off just because your mid laner can't show up this week. It gives greater flexibility to a team's play style. Subs with different champion pools can rotate in and out depending on opponents to maximize the team's strength, capitalize on a particular enemy weakness, or simply avoid a slump. More than anything deep rosters give more effective training, having another player there to bounce ideas off of, to train specific match ups, and test builds against is an incredible resource many teams go without.
While LCS teams may have mostly ignored the results of the EG experiment change is coming. The next season of OGN will see them move to single team 10 man rosters, a change that if handled properly could strengthen the Korean scene. Instead of splitting focus between two teams with two play styles they'd be able to assemble a ferocious group of 10 that fit seamlessly together and constantly vie for the starting position. Closer to home Team Fusion has commenced another grand experiment like EG's with a 7 man roster living in the house which so far has net them 5th seed in the expansion tournament and a strong chance to make LCS.
