I would argue that if you're just starting out, grid hotkeys are more efficient since you can memorize either which key it is or where on the unit-card
the button is located (visual cues tend to help memorization) - and if you don't know the hotkey, you can find it without mousing over the button.
Also, all the keys are on one side of the keyboard, meaning you won't have to move your hand as much.
That being said, the vast majority of players use the default keys either because A)
They don't know grid exists , or B)
They are already used to the default layout (grid was not an option in the beta), and/or were already used to the very similar layout of Starcraft 1; there really is no benefit to them learning a new layout if they already have the default hotkeys memorized.
Most competitive players would fall into this category.
gaming.stackexchange.com/users/2578/BlueRaja - Danny Pflughoeft
Another point to consider in favor of grid hotkeys is that in many cases you don't have to learn a separate set of keys for each race.
For instance: Z-Q makes a base in any race.
Z-W makes a "Vespene Gas thing" in any race. Z-E makes a Terran Supply Depot or a Protoss Pylon.
(Of course, with Zerg you have to make an Overlord instead.)
Z is the normal build key in any race, and X is the advanced build key.
Using T to attack feels weird at first, but in a few days you get used to it.
gaming.stackexchange.com/users/2831/Kyralessa
It's mostly just preference.
You'll find that the standard set of hot keys is as efficient as the Grid.
The majority of the hotkeys, are left aligned after all.
Day9 and most commentators probably use the standard grid hot keys because they started playing in the beta--before the grid was an available option.
That and the fact that most Broodwar and Warcraft 3 players are used to the "Attack" and "Stop" hot keys being in the traditional spot.
It would be pointless to switch or "retrain" on grid layout when the benefit is very little.
gaming.stackexchange.com/users/1919/rysama
It's really up to you, both settings can be done with the left hand
so your right is free for mouse work.
If you want the hotkeys to be the same across races more of them will be with grid mode.
Otherwise the keys are based on the name of the unit/action/upgrade/building/etc.
gaming.stackexchange.com/users/2659/CloudMeta
As someone who uses a Kinesis Advantage keyboard, Grid layout is really my only option, anything else
and I have to take my right hand off the mouse and place it on the keyboard.
As for the control groups issue, I use a Razer Naga Epic mouse
so I have all my control groups on the left side of my mouse as thumb buttons.
gaming.stackexchange.com/users/22764/Wraithan
Certain schools of thought teach that the layout for standard is efficient when combined with the overall macro combinations one normally would use for team games.
Grid confines your keyboard use to one side thereby allowing your left hand to perhaps rest on a single point pivot.
This would allow laziness to creep into your adding to sloppy control group use or neglecting the higher 6-0 control groups.
Also the camera views etc etc
gaming.stackexchange.com/users/None/
I played a lot of beta SC2 games (as random) with the grid setup and managed.  Since then I've started watching  Day[9]  and some other commentators who seem to refer often to the default hotkeys in their recommendations for, eg., keeping drones building at all times.  My question is: to what extent are hotkeys mere preference?  If we took a random sampling of competitive players, would roughly 50% be using the grid layout, or would it skew for some relevant game-play reason?
Which Starcraft 2 hotkey set should I use?
gaming.stackexchange.com
<starcraft-2>
gaming.stackexchange.com/questions/5886
gaming.stackexchange.com/users/2927/ladenedge
Grid hotkeys are more efficient as you can memorize which is which. You don't have to leanr a separate set for each race. Wether or not you are just starting to play, grid hotkeys are the most efficient.
Users agree that grid mode is basically the only option.