It's not often that I find radar game-changing at all, whether it's used by my team, me, or the enemy. I'm no overly-competitive player, I play only random battles to mess with some builds against actual players.
From my personal experience, the elimination of a DD first by a team via radar cannot guarantee a victory. It's more than often that it's the amount of ships a team has at the end, and how healthy they are. Sure, a destroyer alive near the end of the game can benefit the team it's on very well, but what use is it if it's whole team is dead, too?
Yes a competent destroyer captain can turn matches around by himself making use of good concealment and effective torpedo strikes, but you don't always run into those types of players.
The scenarios pointed out to me don't happen frequently at all. Random is a mess and isn't orderly compared to Ranked or Clan battles. Destroyers can be idiotic and die anyways. They can also retreat into safety and move into different parts of the map, which both happens frequently in Random battles.
I'm basing my decisions and choices on personal experience throughout my battles, instead of just being an armchair tactician. Things don't always go as you plan, at least for me. Random is too unpredictable, and sometimes Spotter is better equipped to deal with that.
I'm by no means sternly defending that Spotter is the better choice overall. Both Spotter and Radar have advantages and disadvantages that attract players to use them. I'm only attempting to explain the importance of Spotter, and how some situations that spotter could be more useful than radar.
Being overly-reliant on radar sounds wrong for me, and for my case, isn't a fun experience most of the time. Sure, there's times where I caught broadsiding RN CL's in smoke using radar, eviscerated them and laughed loudly in joy after seeing his ship get ripped apart, but using radar has it's own cons. You are going to have to get close to capture zones to use it more effectively. While I do get close to capture zones indeed to support my allies the best I can, it's less guarantee that I will be able to fire on enemy ships myself to support my team without being blasted early in the game, which is not always fun, which is what games are designed for.
My usage of spotter/radar doesn't have any effect on me at all. I won 50% of my spotter games yesterday and also 50% of my radar games. I'm always sure that I have a radar-equipped ship on my flank to fill in my roles whenever I don't have radar, and I use spotter to allow my team to see enemy ships behind a smoke screen which works well.
Spotter sounds selfish but it really isn't always. It can allow you to hit a key enemy ship easier which the elimination of such will provide benefits to your team. What about taking out an enemy Worcester that's hiding behind the island, radaring all your destroyers? Spotter turns your disadvantage of high arc shells into an advantage. You can hit ships over islands easier that way and I have used spotter for that role and had it work well.
Spotter takes time to get used to and I am fully aware of this. I'm by no means mounting spotter mindlessly and going into a battle. Nor did I just mindlessly unlock the Des Moines and throw it at the enemy, expecting to maul everything. I have studied on how to use the Des Moines as a ship and spotter planes in general. On my first Des Moines battle, I knew well of it's disadvantages of being a fragile ship while knowing it's DPM potential. I got a decent score and helped both the team and myself (and no, I mounted radar on that battle, not spotter). I have a plan set to how to play my Des Moines using spotter plane and I'm adjusting it through experience and knowing when and how to use it.
In short, situations where radar change the game entirely just aren't that common for me at all, and I feel like I could benefit more from using spotter planes to increase both team and personal score. Radar is still a fairly new consumable for me that I'm beginning to understand and use a bit more often. Spotter is a consumable I've used just as much but has lasted for way longer, and has proven itself for ships like Murmansk, Warspite, etc.