I've never used a fiendish dire axiomatic half-celestial undead half-pixie owlbear. Not onece. What a goofy combination!
Of course, I do use templates. Even multiple templates. A lot. The above seems to me to be a dragonsfootesque prejudicial example, but it ultimately meaningless because nobody would use such a combination ever.
Most templates I use in my campaign are one at a time, thank you. Mostly for undead creatures like liches and vampires.
I occasionally use multiple templates, mostly undead creatures, though occasionally not. I used the abberation template on lizardfolk in a recent adventure to make mutant lizardfolk that dark elf transmuters had been making. For one creature, I wanted to make a really out there mutation that would be really scary. Instead of starting with a lizardfolk, I started with an ettin and made him lizardlike by means of the reptilian template, then added abberation to add in that mutant quality and dreadnaught to add a little combat challenge to a party that is several levels higher than he is.
Another combination that I will be using in the coming weeks are the blood lich (aka Sangrolu) and elder vampire template (which is applied to a vampire), creations of my own. One of them is also spellstitched, so yup, that's 4 templates, but 3 of them are sort of designed to work together (both blood lich and elder vampire are templates that can be applied to vampires, and I never make blood liches that aren't old enough to qualify for elder vampire.)
I am running an epic level game right now, and I occasionally use the paragon template to scale up encounters. My first use of the paragon template was a plot-driving danaanshee I needed to make. Most recently I made a paragon night touched controller... I also spellstitched him.
I used to use fiendish and half fiend quite a bit, but not so much recently. I had a roving band of planar fiendish elves who got in the player's hair, and a half-fiend drow bounty hunter party nemesis.
Half-dragon I don't use as much as I gather some people do. I had a plot that revolved around them that never reached fruition, and in my game I have a society of magically engineered half dragons, but those have been more PCs than NPCs. Generally, I am not a big fan of the idea that any dragon can breed with anything, much less want to. For that matter, I don't dig the idea of most half-templates as bred instead of made.
So, in short, I do use templates, occasionally multiple ones, if I feel it is appropriate. But if I do, I always have a reason to.