I do. Almost always. And there are three main reasons for it.
(A) Hassle free prep. Hot water and stirrer, bam!
(B) No waste/by-products (e.g. filter, brewed coffee granules residue, blah blah) - so much less carbon footprint.
(C) A cup of instant coffee has slightly less caffeine content than the same cup of freshly brewed coffee. Good for me, because I have been a caffeine-addict, and trying to cut down now.
So I always try to go for Freeze-dried instant coffee ( especially Colombian, like Juan Valdez ), and not the Spray-dried shit.
Edit : I thank max and the OP for enlightening me about the instant coffee manufacture process. It surely appear that Instant coffee has a lot more environmental impact than simple ground coffee because of more processes involved. I have always consumed instant coffee, but were totally ignorant on this part. Dumb of me.
Thanks to the people directing me to the right way, and I will strike out my second point above.