Because, with LEDs, current varies massively even with small voltage difference. And it goes against manufacturer’s (of LED) recommendations.
Is that necessarily bad?
Yes, it’s not as efficient as driving an LED at 1A or 2A but we do get the peak brightness of an XM-L at 3-4A.
Plus, LED’s just don’t go poof at even 300%+ of their recommend current, specified by the manufacturer.
The HD2010, one of the most highly regarded budget lights, is in fact directly driven at High mode.
What’s wrong with driving at whatever current you want using a constant-current driver? Direct drive has no advantages over constant-current drivers at all.
Nothing at all, except linear driver using AMC 7135’s burn the excess power as heat.
Other than that, using a constant current drivers does enable you to control the brightness. 
Linear drivers aren’t great either. Especially AMC7135 based ones - you can only regulate current in 350mA steps using them, so no option for moonlight and low modes without resorting to PWM.
Yup, I hope we can produce BLF style drivers, preferably a high-efficiency buck driver with BLF’s programmable codes. :party:
Synchronized buck/boost drivers would be best, for those who want to run 1 cell. Buck/*boost* because draining modern 2900/3100/3400mAh cells requires boosting due to them having not-so-high voltage.
Modern cells can’t stay over 3.3-3.4 volts under load?
Anyways, I wonder if we can arrange something with Fasttech about our drivers. /wistful thinking
Nope (at over 2A)
Not totally correct. P=UxI. Different LEDs have different Vf
If someone decides to organise group buy sign me in for 2 or 3 … or 4 
Mouser shipped mine out, Qty 3, XM-L2 T6's. We will see. I got 2 cheapo Sipik SK73 zoomies, 1 with a U2 1A, other with a stock T6. I can do ceiling bounce measurements on these 2, then swap the XML T6 for the new one, interesting comparison then...
I received the 7 xml2 this morning and quickly put them into my dry. I am loving the tint of these leds
33696933 - Wow! That was fast! Dunno about the DRY - single or multi LED? Was the form factor the same for mounting? Height the same? What's the tint comparable to warmer or cooler than a T6 3C? Can you notice brightness diff or can you do a ceiling bounce test w/meter?
Sorry - lots of Q's... You are the first one known to get, see, and install these things... 
the dry is multi (3). it looks similar to a 3c to me
Noticeably brighter or the same?
Ok, maybe this evening you can see how it does in brightness? From the specs for a T6, was expecting it to be at a U3 level - 2 bins higher, probably somewhat difficult to see, though in a 3 LED DRY, maybe it would be more pronounced. Also, there was speculation it will be easier to dedome tham the standard XML's - wondering about that...
Should get mine in a couple of days, chose USPS shipping.
tint wise it looks similar but brighter
Nice! Was hoping it would be noticeable. I'll be getting a lightbox in soon (maybe a few weeks) to do lumens testing from a good source, though got the light meter already. I only got 3 of these LED's coming in, but can do before/after ceiling bounce tests. Interesting to see how it does between throwers and standard lights like a C8. Actually got a brass C8 pill that it would be perfect for - I'm running a U3 1C now in the stock pill, KD V2 driver at 3.8A. If I build up the brass pill with the XM-L2 T6 with the same driver mod, same star, I can swap pills easy - would be interesting.
Crap! should have ordered more - working on mod upgrades now, but 3C tints are not for everyone's taste. Can always order 1/2 dozen more... Not certain about the removal/replacement, but I think you just use the iron w/wide tip and heat the star from below. To put the new one on, apply a little more solder to the 3 lans, then same thing - heat up from below. Did this once to reseat an emitter - seemed to work. I know their are postings on this somewhere here on BLF...
Can you post a few macro photos of them?