Capn Bloggs,
Let me enlighten you to the joys of using visual separation. An example if I may:
Cloud base 5400ft. Inbound on decent LEFT 4300ft (doesn't matter whether it's an instrument approach or not) and you are approaching over head for a sector entry for any approach. I assign you 5300ft, you get visual, I get you in sight and can give further decent and clearance for the approach using visual separation with the inbound. If I'd assigned 5500ft or worse 6000ft you are still in the muck and you have to do a holding pattern before I can give further decent. Additionally by getting you visual I can facilitate climb (using visual separation again) with someone departing who is maintaining under you in the sector entry.
Finally - if you are held up on decent for an approach by a preceding doing the same approach then there's a good chance he's on decent or maintaining a MSA or track LSALT etc which don't seem to come in even 500ft levels. I will always give you as much decent as possible - ie 1000ft above this level!!!!!!
Hope this helps.
RI