Its never 100% because there is always overhead, and you also lose another 5-10% of the total performance due to lower boost clocks.
But yes, some games can scale really well and the performance you have under the hood at that point is pretty awesome. I just like my gaming hassle free... and from my experience of owning 2x GTX 660 back in the day, it was anything but that. It was hot, it was loud(er), and I did come across several games that didn't get proper support. If you play a wide variety of games, stay FAR away. If all you play is triple-A stuff, then the experience will be a bit better. Another consideration is the balance between VRAM/bandwidth and core power. The latter doubles, but the first does not and without proper frame capping / sync you will have stutter.
If the choice is ever between 'a single top tier card' versus 'two of a lower tier' - always go for the single. If its about adding another GPU after a while because you feel you lack performance, be very wary of your target resolution and whether the VRAM can handle that.
EDIT: since DX12 developers are supposed to add multi GPU support themselves for the most part as well. This doesn't help either... Nor do multiple APIs (we now have Vulkan as well, and DX11 is going to be around for awhile too).
Everyone's experience tends to differ when it comes to SLI/Crossfire.
Personally, based on my past experiences with it and what I see coming down the pipe I have a different appreciation for SLI than you do. I, however, have a similar look at the future of it.
I ran SLI with the following cards over the past dozen years or so:
7600 GT
8800 GTS 640MB
8800 GTS 512MB
GTX 280
GTX 570
GTX 980Ti
Rarely did I run into issues after the 8800 GTS 512 cards. Before that, there were a lot of games that had poor support or just really odd issues that no driver would fix. I don't recall the 3rd party program that I'd have to run and then it would allow a lot of tweaking of settings (kind of what you see when using the Manage 3D Settings in the Nvidia Control Panel these days) to help fix odd issues. One issue I vividly recall when running the 8800GTS cards in SLI was during The Witcher - light sources in the game would brightly shine through walls. Torches or the sun would shine through background drops (trees and such) or through walls in buildings. If I disabled SLI the issue went away. I had to use the 3rd party program to fix the lights bleeding through when SLI was enabled.
Sure, I'd come across a game or two that just wouldn't run with SLI enabled. No big deal, just disable it and be on my way to gaming after about 30 seconds. Some games didn't see performance increase with SLI. Some games didn't utilize SLI. Some games saw great boost in performance with SLI while most games gave a good/decent performance increase under SLI.
Sure, the top card ran hotter - usually 10C on average. And the power draw was higher, but I loved SLI. I could buy one high-end card and usually after 6-8 months later I could pick up a second one at a much cheaper price point. Then I'd have a system that would outperform a single top-tier card or a system that would still provide solid performance if SLI support was lacking or not working (which in my experience was a rarity).
With DX12 on the horizon (let's face it, games aren't flying off the shelf that only support DX12 yet) and having the developer being responsible for enabling multi-gpu support.....I just don't see the developers naturally going this route. Plus with only top-tier/high end cards supporting SLI these days, I don't think most people should worry about going SLI in the future. I believe that the 9xx series was the last series to make good use of SLI.