I have fitted a ‘topping lift’ or masthead line and use it for the following reasons:
1. Hoist a secumar automatic inflation cushion, which I use as additonal masthead bouyancy (see inversion topic).
2. Hoist a mast head light if required for anchoring or night sailing.
3. Support the back of the boom when using a boat tent, admittedly as belt and braces with a boom crutch but it does mean that the boom does not drop on you if the crutch slips.
I would not cruise without one as it has a variety of uses. If motoring or rowing with the sail up I scandalise the main, as Kieth doe,s using the reefing line (you have to remove or lengthen the kicker).
I prefer this arrangement as it leaves the main halyard free for other tasks (support front end of boom for boat tent if sliding gooseneck misplace) or just out of the way. It can of course be used as a back up topping lift (everything in a W should have two uses….)
As to having the sail in the way with the boom held up – just roll it up and secure it to the boom with bungee ties (simple enough to make up yourself) – there is no need to remove it from the boom. This looks neater, keeps the sail clean and out of the way (especially from fish) and will reduce the likelihood of damaging a sail batten in the bottom of the boat. This also keeps the sail out of the way inside the boom tent.
A topping lift is easy to fit with a cheek block at the top of the mast and a cleat on the mast or tabernacle and provides a multitude of uses (as well as drying washing/clothing). Very much worth it in my opinion but mostly for supporting masthead bouyancy.