They're not useless, in fact they're a handy guide if you have them, but the only tell a tiny part of the story when it comes to how useful a turbo is. For this reason, basing your turbo choice on what you see on a compressor map is ridiculous, and just shows that you, despite no doubt feeling smug that you understand how to read a compressor map, also shows you don't really know how engine tuning works...
I've actually seen a whoooole lot of people choose not to buy a turbo because there's no compressor map publicly available (or just doesn't exist). And considering pretty much no aftermarket turbo has a map available, and not even all OEMs publish theirs, this is pretty ridiculous.
The fact that the vast majority of the fastest and most powerful tuned turbo engines on this planet runs turbos with no compressor maps available for them is a hint at how "Think they're clever but in fact are stupid" (what's a good term for that, barring dickhead?) so many people are.
So what are compressor maps good for? Well, lots of things. They tell you the maximum (roughly, it's not gospel, most can do a bit more) airflow of a compressor. They tell you the maximum (roughly again, most can do more) pressure they can achieve. They also show you what sort of pressure and boost they're most efficient at, though if you're aiming for the most efficient point you'll be giving up both spool and power compared to a different spec turbo, which is a bit daft.
As the above sounds useful, why am I saying they're not that important? Well, while it's not as accurate as a compressor map, compressor wheel inducer and exducer sizes are a fairly good indicator of these things too. How much a compressor will flow is indicated by how big the inducer diameter is, and how much pressure it's happy to push is generally indicated by a bigger exducer diameter compared to inducer.
The another big reason you can't rely on them is because, as mentioned, the vast majority of the worlds best turbos simply don't have compressor maps available (Only some Garrett, some BorgWarner, and very very few from others), so you'd be likely hugely missing out on the best turbo for you if you went purely by maps.
Also, and this is a biggie, thanks to the wonders of the internet, plus the experts who create and sell these turbos, you're hugely unlikely to be venturing in to uncharted waters by using a turbo, and a bit of research can almost always find out how much they flow, how much boost they can produce, even without seeing any hint of a compressor map.
And this brings me on to the main reason Compressor maps aren't that important- ALL they show is the compressor side, HALF the turbo, and on most applications the less important side...
Compressor maps don't show, in any way, shape, or form, the number one question for 99% of people- How a turbo SPOOLS. Not only that, but it also doesn't show if the turbine side is far too small for the application; something that's often ignored, but is a HUGE factor on reliability and power.
An overly big turbine side won't spool well, or indeed at all- Compressor size has very little effect on spool.
An overly small turbine side not only can restrict power and usable rpm, but high-pre turbine backpressure causes huge reliability issues that wouldn't happen with a correctly sized turbine- What goes in has to come out! Check out my earlier feature HERE regarding backpressure for more info on this...
So to sum up...
Compressor maps = Useful yes, but vital, hell no.
Choosing a turbo via a compressor map = Stupid, as ignores all the main factors.