ANSA, what I was descrbing wasn't taught at the college that I went to - only by others during OJT. I guess it's only likely to happen around an aerodrome rather than en-route (certainly in the UK). The most common reason in my experience is when the TA is low, say 3000 ft, and inbound vectored traffic is being separated from traffic that is holding at relatively low level. The easiest way to be sure of getting vertical separation is to put the inbound on QNH a little earlier than usual and use altitudes above the TA.
It's one of those things that I should probably explain more to the pilots but I generally don't have the time. Occasionally a pilot will query it but most don't - perhaps this is because in the UK we have different TAs in different areas - 6000 ft just down the road at London, for example - so the pilots are not unused to hearing the sort of altitude assignments that I might use.
Like I said, I wouldn't argue that it's good practice but I don't think there are any rules against it and it sometimes saves lots of mental gymnastics if some of your aircraft are flying close to TA for some reason.