True about flexibility being important, but I don't think 6th man matters much. They have same system in LoL (at least some had) and most don't even know who the 6ths are. Like think about the best western LoL team world has seen (G2) had like Promisq and Pinoy as 6th men? If you don't know: they wweren't very good. Same as F1 teams have reserve drivers. I think its going to be pretty similar to that and you will see situations like when Otom was on Fnatic.
Some teams will use this properly tho, but my prediction is that many 6th players are just some tier 2/3 who just sit on bench most of the time. Obviously you have games like Overwatch where having more than 6 players actually helps a lot. But I doubt that will happen in Valorant.