You didn't say low end SSDs. On top of that a USB 3.0 flash drive at those speeds costs more per GB than a top end SSD.
The SSD in the surface pro reads at almost 500 MB/s. A flash drive is not comparable nor is it practical as a solution for extending its storage. You're better off buying the 128GB model.
It's not practical for extended the storage for permanently installed app space, true.
But for storage space of rarely accessed media, documents, etc?
I think you are misunderstanding speed differences.
And have completely forgotten where we came from, what we had. We were lucky to have 80MB/s platter-based drives on 100/133MB/s interfaces.
When SSD came around and become somewhat affordable for "the masses", 300MB/s was consider supreme, if that was even attainable (it was still out of my reach for cost per gigabyte/practical measures). It was common to find drives that would dip down below 250MB/s for certain measurement methods, like combined read/write or different sector sizes, etc etc.
In short, as time marches on, USB 3 thumb-sticks will continue to get faster. In the past year, iirc, they've already moved from roughly 100MB/s consistently, to over 250MB/s in some metrics and what otherwise appears to be 200MB/s consistently.
Look, I'm not looking to debate the matter or turn this into some thread derailment.
IMHO, media/document storage on a 64/128GB USB3 250MB/s thumbdrive is perfectly acceptable. I could reach out for such even when I can get my hands on the 128GB Surface Pro. I won't, not right away, as I'll wait for prices to get into the more reasonable category.
But it's also worthless to compare to SSDs for pricing - that's not how thumb-drives should be compared, because the utility is of a different nature. And we can't upgrade the SSD in the Surface, though it may be "hackable" - perhaps it uses mSATA and it's locked away simply because the device itself is "sealed." mSATA drives are hard to purchase as a consumer, but they can probably be found. We'd have to wait for a good teardown.
If you think 250MB/s is slow for a hard drive, especially one without the OS, you're simply out of your mind. Even as a boot drive, that speed would be extremely beneficial and a major time saver. For anyone with a HDD, they'd take it in a heartbeat.
Of course it won't compare to recent-model SSDs and/or new firmware releases - I don't think thumb-drives will ever match the top SSDs of the same launch window - not enough silicon to write to simultaneously.
But accessing large files that aren't often accessed, a bunch of MP3s, movies, documents and photos and other media, especially, say, graphical files? Unless the dongle makes working with the thing that much more difficult, it would actually be BETTER to have external storage. You could get a full proper external drive, but that's much larger in comparison, but you can even get an SSD and have the same speed, or many will get physical HDDs for the storage benefit that they use from time to time.
But honestly, it would be best to take a little effort to save on the write cycles for the SSD.
It'll be wonderful once SD cards have a good speed/performance increase. UHS-1 is great but UHS-2, and the actual silicon to provide that performance sustained and consistently, is needed for it to be a welcome addition for anything but rare media use.