I have two camera bodies in my bag: A Canon 5D and a Canon 20D. As a rule, the 5D is my primary camera and the 20D serves as my backup. That aside, I sometimes decide to shoot with my 20D.
We all know the 5D sports a full-frame sensor on board while the 20D, like all Canon's double-digit-DSLR-cams, feature 1.6x cropped-sensors. The exception is the new 7D: Single-digit nomenclature yet also with a 1.6x cropped sensor.
When I go with my 20D, it's not to obtain further "reach" in an optical sense, i.e., to somehow magically turn 100mm into 160mm. That doesn't happen. You don't suddenly turn a lens into a longer lens because you're using a cropped-sensor camera. No-sir-ee. Technically and optically, you don't get more "reach" with a 1.6x cropped sensor. It only seems like you do because of the crop factor. But the fact that it "seems" like you do is what causes me to, sometimes, reach for my 20D.
When I go with my 20D, it's also not because I suddenly decide I want less pixels capturing the reflected light. More pixels can be a good thing but pixels aren't everything. For the most part, the 20D has sufficient pixels for most of the work I perform. Yeah, the 5D has a superior processor and other technology that trump the 20D but that's getting way too technical for me to write about and, frankly, those aren't huge factors when it comes to capturing good images, especially when shooting the kind of stuff I normally shoot.
Back to "it seems like" you get more reach...
While, technically, more reach from your glass does not happen with a cropped-sensor camera, what you see in the viewfinder appears as if you do. Sure, I could later crop an image captured with a full-frame sensor to match the crop obtained with a 1.6x sensor but cropping in post is not always the same as framing in production.
I prefer to frame my images, when shooting, in a way that most closely resembles what the finished image will appear like, composition-wise. Perhaps it's a product of my many years shooting with video cameras? You know, where there is no cropping later on, in post. Regardless, I try to avoid excessive "loose" framing other than with a nod towards text and graphical elements that might later be used with the images in ads, or for DVD cover art, or for other uses.
I prefer framing in-camera, I suppose, because there's something spontaneous, something in the moment, something in the way the model moves me that affects my framing and composition. (Sorry if that vaguely sounded like the lyrics of a Beatles tune.)
Often, there's a rhythm a photographer and model get into when shooting. I'm fairly sure many of you have experienced this. It's an awesome thing when it happens! Maybe not as awesome as sex but pretty cool, nonetheless. And that rhythm, leastwise for me, affects my framing and composition. If, instead, I simply relied on post (only) for cropping my way to finished composition, that composition might sometimes, certainly not always, suffer as a result. Why? In post, the rhythm with the model no longer exists and that something that was special in the moment is now history in terms of how it might have affected my framing and composition... if that makes sense.
The gratuitious eye candy at the top, with me sitting between her legs (great seating, btw) and her shining a light on me, is Kayla. I've probably posted this pic before but, due to my recent move, I'm still living out of boxes and my desktop computer, with its hard-drives containing so many images I've shot, remains unavailable to me.