What would you say it is that makes a game with Flash more interesting than a game without it? I can understand that having anything with flexible use, a powerful effect, and a long cooldown can lead to some interesting use cases, but most of the time (even in LCS and Worlds), I mostly see it used as a gimme or a crutch. Accidentally stepped out of position or got caught out? You've got Flash for that. Enemy managed to get away from you with a sliver of health and you're not quite close enough to use your ult? You've got Flash for that. Dude you're ganking saw you coming but you still want to get that kill? You've got Flash for that. Didn't see the jungler coming after they took the time to coordinate with their ally but want to not die anyway? You've got Flash for that. Enemies on your ass and you aren't close enough to your tower to fend them off? You've got Flash for that. Overstayed your welcome under the enemy's tower and know that the shot after this one will kill you? You've got Flash for that, etc., etc., etc.
While there's no doubt that there's pockets of depth that Flash brings to the game that wouldn't be so universal if it didn't exist, it honestly feels like something that dumbs down the game as a whole - since for every cool and strategic use for it, there are hundreds of get-out-of-jail-free cards. The "big plays" involving Flash that Riot and the eSports community talk about are more often than not really, well, flashy displays of split-second reaction time (Thresh flashing through an Ashe ult and hooking her immediately, for example), rather than jaw-dropping moments of strategic and tactical finesse. There's nothing wrong with a little shock and awe, of course, but it feels a bit misaligned with the strategic depth that you guys are constantly seeking to add to and refine within League of Legends!
I guess in general, I'm more inclined to agree with Zileas on this one. Not to say that League isn't already fun with Flash as a mandatory summoner spell, or that the inclusion of Flash drastically reduces the possible variety of champion types and playstyles, but I do think that a game without Flash would lead to a playerbase that ultimately improves at the game at a faster pace (since lessons in the form of grey screens tend to be more effective than the slap on the wrist of having to wait a few minutes before you can safely screw up again), a game filled with greater and more impactful gameplay consequences (especially in professional play), and a competitive roster that isn't limited to, more or less, who can optimize this really, really powerful tool the most.