Here's my thoughts.
I hate the "ergonomic" keyboards. My hands don't work that way. I don't always hit H with my right hand, etc. Being forced to use one of these keyboards invariably slows me down.
As a long-time PC user, I've never had an issue with using modifiers in hotkeys. However, modifiers slow down the pace. If you're trying to spam several keys in combat, you can't be expected to quickly change shift/alt/ctrl states between them, and if you're trying to use the mouse at the same time it will also mess with your mouse clicks. These are huge arguments
for the use of additional keys such as extra mouse buttons, extra keyboard buttons (a la G11, G15, etc), or a separate input device (G13, X-Keys, etc)... and so I have to disagree with "not losing anything"
Now, as for the G15, I really don't mind moving my hand to the side because I've already committed to having my hand there during combat, and everything I need during that time is based around that area. I will usually use the mouse to move (as opposed to moving back and forth for wasd during combat; I usually only move my hand back to wasd when out of combat) or adjust as needed. The G-keys are close enough to use any of the modifier keys if I need them, and I build my key layout from the bottom toward the top for easy reach. Here's an example of what I've done:
http://www.lavishsoft.com/wiki/index.php/ISBoxer:Lax#Logitech_G15_Keyboard. Notice that Follow, Assist, and DPS are right there are the bottom -- that's where my hand will rest during combat, and I will pretty much spam those keys. If I were to update the image, my Interact with Target is now directly above the Assist key, so I would spam that and DPS instead of Assist. Love my G15.
(Note: not a big fan of the G15 v2 with only 6 G-keys, and I would prefer an 18 G-key G11 over the 18 G-key G15 because the LCD is useless to me)
On to the G13. When I'm boxing on my laptop, I use the laptop keyboard along with my G13 (whereas I use G15 with my desktop). I like having additional "extra" (non-keyboard) buttons within reach, but I miss having easy access to the alt, shift, and ctrl modifiers. I could use one of the thumb buttons on the G13 for a modifier (
How to use any button as a modifier key), but those buttons aren't quite as comfortable to me. So I lose out on a little bit of extra functionality on the G13 compared to the G15.
I also use a Razer Naga mouse regardless of whether I am on my laptop or desktop, and I use the standard 1 through = mapping for the 12 buttons on the side. I use them instead of moving my hand back to the number keys for something mundane like pressing 1 or 9 for pulls.