Taking exhaust or ignite can often be the difference in getting those early kills, which mean far too much in league.
I find just the opposite to be true. Once the early game passes, Ignite's usefulness drops dramatically, outside of facing a few key champions such as Tryndamere, Swain, Volibear, Mundo, etc. I would rather have Cleanse, on most Champions, which stays useful throughout the game, than Ignite, and get a couple more kills early on. Not to mention that Cleanse counters Ignite, and has the same CD, making it even more superior in my opinion.
Problem is that every bruiser nowadays, and half of the ad carries, have a gap closer, making flash required. I've experimented with taking ghost instead, but for the most part ghost cant replace flash unless I have a reliable escape.. And nothing really compares to am's blink.
It's actually just the opposite, I believe Flash helps people with a gap closer MORE than it helps those without.
Let me give you an example: Everyone says Veigar is useless without Flash. They say he would just die instantly without it. Yet Flash is ALSO a great counter to Veigar. Without Flash, even if Veigar misses his stun, as long as they are caught in the middle of the ring they have to stay there for the duration, or be stunned. And even pseudo-blinks such as Grave's Dash or Jax's Leap will get stunned by hopping over the barrier. Yet people with Flash can easily hop out of it. Which means even if Veigar is PERFECT at landing his stuns, and always hits somebody, the people who he didn't hit, on the inside, can just Flash over and kill him, or Flash away.
Sure, Flash is also great for Veigar for getting away etc., but I think the buff to his stun would be tremendously more impactful than Flash's ability to help him escape every ~300 seconds. Not to mention that if Flash were removed from the game, it would also increase his own survivability as well.
In terms of champions who have built in gap closers, there are typically already problems with these Champions anyway. Elise, Khazix, and Jarvan, 3 Champions with screen-wide gap closers, are among the best Champions in the game, and all of them need nerfs IMO. Ezrael, who was super powerful for a long time because of his powerful, built-in Flash, just got a pretty big nerf a few patches ago. Plenty of Champions with crazy gap closers are already overpowered, and that's a separate issue, that the Summoner Spell Flash just mitigates. If anything it would bring these problems into the light more clearly.
And before you go saying that pseudo-flashes are not what makes carries strong, listen to the opinion of one of the best ADCs in the world (some consider him the best ADC) who says that any ADC without a built-in escape (or massive range) is ABSOLUTE GARBAGE, and not worth playing in competitive play. He also says that EZ's escape is what made him so good in the first place, because he has no clear weaknesses. This is what I've been telling you all along. Once again, this is a problem with the Champions themselves, not with Flash. Though if anything, I think Champions like Varus and Draven would be much BETTER if Flash were removed, because you couldn't get away from them as easily.
http://www.gamespot.com/league-of-legends/videos/know-your-role-ad-carry-with-doublelift-lcs-week-3-6404372/And yeah, power treads are indeed the common mans boots.. Just as flash is the common man's summonerspell.
Finally, Treads are taken on about 50% of everybody, and have 3 different abilities which are all significantly different, and apply to different heroes, almost like making it 3 different items. Flash is taken on 92% of everybody, and is used the vast majority of the time to escape a mistake of bad positioning. These two aren't even comparable. For one thing, Treads aren't bad for the game. They actually have the SLOWEST MS of all the boots, meaning a player who takes them has to have superior positioning than someone who doesn't, where Flash rewards bad positioning.
