I have heard again and again that using the digital viewfinder on your SLR is a big no-no and that you should always use the optical viewfinder when framing a shot.
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Pro photographers do not necessarily always take photos via the optical viewfinders of their DSLRs. Due to the phase detection system of the optical viewfinder, many photographers when shooting subjects that require very quick focusing, will use this method as it is quicker than the contrast detection used when composing with the LCD screen.
I do a lot of staged commercial shoots. These are all studio based with no moving parts and therefore, speed is not required by me and I use the live view screen to set up my shot. Sometimes, it can be upwards of 2 hours for me to set up all the fill lights, the reflectors, the diffusers and black cards before I make the first exposure. Having the tripod mounted and the live view on, allows me complete control over how the setup needs to look prior to clicking. I very often go one step further and use a laptop screen tethered to the camera and use that as my live view which makes setup even easier.
On the other hand, if faced with motor racing or wild life, the preference will always be to use the optical viewfinder as it will allow me to follow the subject more naturally as I pan and swivel my head with the camera becoming an extension to my eye, plus, as mentioned earlier, it is a faster method of focusing and also provides a larger view of the subject when compared to a 3inch LCD screen.
1.) First of all the digital view finder drains a lot of battery.
2.) The SLRs are most efficient when used optical view finder. Its designed that way. e.g. fast autofocus. The SLR works by splitting the light into three parts one for optical view finder, one for autofocus ring and one for lightmeter. For using digital view finder the mirror is raised and the splitting doesnt work. You can google this for details. In short using optical view finder is faster.
3.) On some cameras shooting with digital and optical view finder can alter results, as electronic view finder tries to adjust exposure so that you can see clearly.
4.) If you are low on battery you can compose your shot before actually turning the camera on.
5.) I would say habit is also a contributing factor. But wont argue with this.
Update: After some discussion, it looks like my answer was not reflecting what I was trying to communicate, so I'm updating my answer as follows:
I know some pros who feel comfortable using Live View. One user of Nikon D750 is my friend and he prefer live view, especially to capture landscapes and natural scenes. And with view finder, he prefers it for birding and sports kind of things. So wherever it is comfortable, they would use it. See his photography here: https://www.flickr.com/photos/mohsanraza/
In addition, I've also come to know about Canon 70D and 7DmkII having more advanced auto focus in Live View and so providing photographer more comfort and ease in their compositions.