Assuming you're using "service loops" in the sense of allowing enough
cable to allow a server to slide out while running... usually in copper
building wiring, the term loosely refers to excess cable or whathaveyou
stuffed back into the conduit/cavity/box to allow for the fixture to be
pulled out and worked on.
When you've got a dense rack (think something like 30 1U servers, with a
minimum of 4 x Cat5/6/etc to each one), "service loops" are a great way
to significantly reduce your airflow. Think about how far you have to
pull a server out... is anything significantly less than 30" deep these
days? That means a lot of wire to store. When it isn't mission critical
that downtime be minimized to the second, it changes the perspective on
whether or not you need to be able to pull equipment while having it
still running.
So, if you really need the capability, an alternate method for providing
"service loops" is to simply swap out cables. You disconnect the precut,
swap in a nice long cable. Pull out your server. You lose connectivity
for a moment or two, but don't need to make arrangements for extra feet
of cable per each 1U.
Each situation will have tradeoffs. Pick appropriately, as always.
... JG