We never talk about bels or centibels or millibels, everything is always expressed in decibels for some reason.
a) Are there any other metric units that have this same strange quirk
and b) Are there any other units where deci- is (one of) the most commonly used prefix? I know deciliters are used occasionally in Europe for certain things but it’s rare to think of any other units where deci- is ever used.
You’ll never see area measured in ares, or kiloares, or centiares, only in hectares. I half suspect that the “are” was back-formed because they wanted a unit of the same order of magnitude as “acre”, and they put a “hecta” in that unit because it’s the area of a square 100 m on a side (making it 10,000 square meters).
Does anyone use the “Stere” (one cubic metre)?. Cubic metres and centimetres (or "cc"s) seem to be the only units commonly used for volume. If I’m not mistaken, one stere (or cubic metre) is equal to one million cc’s.
(one light year is equal to 9.46 petametres) 
(apparently the stere is used in Europe for volumes of firewood)
The water company, when they bill you. I doubt you only used a few cc…
My water bill in measured in cubic feet.
Not metric, but the CGS unit centi-poise is commonly used for kinematic viscosity. It’s abbreviation is cP. It maybe because the kinematic viscosity of water is around 1 cP at ambient conditions.
The milli-Helen is the unit of beauty sufficient to launch one ship. I have never heard of any other prefix used with Helens.
I don’t think degrees of temperature are particularly useful by any units other than centigrade. But I didn’t come up with that aphorism: 1643: Degrees - explain xkcd
Huh, I had no idea a centigrade was 1/100th of a gradian!
CGS units are metric. The “Metric System” is an umbrella that encompasses CGS, SI, and a few other related units. For instance, “mm of mercury” can be used as a “metric unit of pressure”, even though it’s not CGS nor SI.
Come to think of it, that’s another one for this thread. Nobody ever measures pressure in centimeters of mercury, or micrometers of mercury, or whatever. If you’re in an application where mmHg aren’t good enough, then you’re probably using pascals (possibly with scientific notation or a prefix).
It’s 285 degrees Kelvin outside now - a high of close to 300 degrees later today. 
I don’t know if this is officially part of the metric system. But in radio frequency (RF) engineering, power broadcast measurements in units of dBm (decibels referenced to one milliwatt) and dBw (decibels refernced to one watt) are common:
10log(1 watt) = 0 dBw = 30 dBm = 10log(1000 mw)
There is an Imperial measurement that is used worldwide - in metric regions as well as non-metric.
The knot.
Aircraft and ships seem to always use it. 1.15078 mph. 1.852 km/hr.
Every audio system I have ever bought was in stereo.
Microns of mercury is a common measurement for intermediate vacuum systems. My Kinney KC-5 can pull down to 25 microns. (Anyone need a vacuum pump?) In a vacuum system in that range we usually said micron instead of Torr. Lower then 1 micron (10 -3 Torr) it was always Torr. But seldom milliTorr, we always used the -3, -6 or whatever. A shorthand way of referring to a high vacuum system might be, 'It can pull down to the -6 range".
Dennis
Degree is not used with Kelvin.
“ The 13th CGPM changed the unit name to simply “kelvin” (symbol: K).[13] The omission of “degree” indicates that it is not relative to an arbitrary reference point like the Celsius and Fahrenheit scales ”
This is related to 360 degrees in a circle (does that count as “imperial”?), each of which is divided into 60 minutes. That raises the question if anyone uses “millidegrees”. Milliradians, sure. Minutes and seconds, definitely. But who uses centidegrees or some other decimal fraction?
Every audio system I have ever bought was in stereo.
What does that have to do with the OP’s question?
Incidentally, as DPRK pointed out, the knot is not (pun intended) an imperial unit. It is one nautical mile per hour, and the nautical mile was defined as one minute of arc (1/60 of a degree) along the Earth’s meridians (or one minute of difference in latitude). This has the charm that differences in coordinates can be more easily converted into distance in nautical miles. It is also the reason why nautical charts do not indicate a bar scale. Instead, they indicate degrees and minutes of latitude and longitude, so if you want to know how much a nautical mile is on your map, just look at the left-hand side where you can see the latitudes.
True, and the way area is commonly reported in the media really bugs me. I hate it when I read news like “766,000 hectares of forest were destroyed in the 2018 California forest fire season”. Give me the same data in square kilometres (7,660), and I can easily visualise it - as a rectangle 76.6 kilometres one side and 100 kilometres the other, which are distances I can relate to. Much more intuitively graspable than 766,000 hectares. The only thing that’s worth than that is if area is stated in “football fields”. For some reason, journalists seem to believe their readers are too dumb to understand square kilometres, but can intuitively relate to an area of so-and-so-many thousand football fields. And I have no clue how many square metres a “journalistic football field” is supposed to be equal to.