Let me explain the title of my post.
I've been doing a lot of location work outdoors in various conditions and times of day. I also did an experimental shoot the other day with a single Nikon SB-900 Speedlight. It was a disappointing nightmare to say the least.
I'm not knocking the use of Speedlight's and I'm certainly not saying you can't use them because many people surely do, but I do find that there are many limitations that I find unacceptable for my work.
This isn't anything new that most of you don't already know but rather than just conceptualize about it, I'm putting in real world terms.
I used a 46" Softlighter II as my main light source and since I was shooting roughly from the waist up I needed to keep the Softlighter about 4-6 feet from my subject depending on the angle and position of the light relative to the subject.
First I tried using Nikon's Advanced Wireless System (a subset of CLS) just to see if that would work and surprisingly it did work even in bright midday Sun but (and it's a big one) it absolutely required line of sight and made it impossible to position the main light on the opposite side of the camera's built-in flash when the camera is turned to portrait orientation. The sensor on the flash couldn't see the camera's built in flash which meant that the light had to always be at camera left. I could get the SU-800 controller for better range but it doesn't completely solve the problem.
When conditions were right the wireless TTL worked well but trying to underexpose the backgrounds using TTL FP (HSS for Canon) mode was a power sucker and only worked if the flash was very close to the subject. Once I backed off the flash I was underexposing the subject and any FEC applied was useless because the flash was already at full output and struggling due to the 1/2000s shutter speed required to bring the background down in exposure. If I wasn't concerned about the ambient background then I could disable the FP mode and get more power out of the flash, which would allow for smaller apertures or perhaps pulling the light back more but my backgrounds would be overexposed and clipping all over the place.
Based on those tests I knew that using Pocket Wizards and working in manual mode wouldn't change much other than to give me a constant exposure instead of the minute changes that may occur using TTL.
There are also limitations to the size of the modifier you can use with a Speedlight as well as the selection. There are solutions but none are provided directly from the manufacturer and must be purchased separately from third parties. I can mount a Speedlight to a 69" Octa but it's really not a great idea and an awful waste of flash power. You'd more than likely always be at or near full power and will absolutely require an external battery like a Quantum.
Even if you have a Quantum, if you're shooting at full power then you have to be careful of how quickly you fire due to overheating issues in hot shoe flashes. Some flashes like the 580EXII and SB-900 have a built in thermal shutdown and you can be assured that if you fire off a couple of shots at full power and then a few more and a few more, your flash is either going to tell you it's about to shut down or it will shut down. Disabling the thermal shutdown can be done but not really advisable if you're really planning on working the flash much harder than intended.
Of course you can start getting into setups with 2, 3 and 4 flashes firing but that's not for me. I know that for many people it's a great solution but I decided that if my lighting requires more than a Speedlight or my 120J then it's time to get out the big guns.... the Elinchrom 600RX's.
Once again, let me stress that I'm not trying to tell anyone else how to work or what works best for them. We each have a different style, preferences, and are willing to compromise in different areas. For me, I'm just not willing to compromise when it comes to lighting so rather than try to make a small flash do the work of a studio strobe, I'll take the pain in the arse that goes along with moving gear around to have the lighting I need.
When I compare my experience with Speedlight's to the same scenario using strobes and large modifiers I can't because it's not even close. Strobes made my life easy. I had more power than I needed, my recycle times were fast and kept up with me, I had no trouble using a 69" Octa (I borrowed one from a friend and fell in love immediately) and I was able to distance the modifier from the subject and still get beautiful soft light, even the full length shots. I used a CPL and 2 stop ND to reduce the overall amount of light reaching the sensor. The fact that I could combat the Sun for my 2 to 3 stops of background underexposure and was able to open up the lens and shoot at larger apertures made it a cakewalk. 
I adjusted power of the strobe as needed from the Skyport transmitter and even though I could do that with the Nikon wireless system it requires menu diving if you're not using an SU-800. The large modifier also allows for a larger subject area before the light falls off or natural vignetting occurs. When I use smaller modifiers I have to adjust the modifier more as the model moves or changes poses but the larger modifier gives the model more freedom and allows me to maintain the workflow and vibe.
I'm sure some will be disagreeing with me and that demonstrates how once again there is no one way of doing things and your lighting decisions will be based on your own criteria such as cost, convenience, weight, power output and consumption, indoors or out, time of day, type of shot, number of subjects, etc.... you get the idea.
) Things like softboxes etc. didn't even exist. It was more choosing between hot lights and strobes. (Packs and heads.)
