it is very practical, but it will decrease your system water volume and over the long run (years) it adds up to a lot more work. most importantly, you cant show off those pristine, shiny, tumbly reactors on deck, naked in your face (less style points). lotsa guys put reactors in a smaller container above the sump or to the side.
time yourself. trust me its a few more minutes maneuvering reactors in / out of the sump, drying off. its also harder to eyeball the reactor to make sure its working. ive known guys who quit the hobby because it was too much hassle to remove the skimmer cup or filter sock. a few inches of clearance makes all the difference when youre at the point in reefing when its not so fun, feels like work and youre on the fence about quitting.