Re: Why use a topping lift
If the main is reefable, the topping lift will support the boom during the reefing process making that job much easier.
The downside of the topping lift is, as you've noted, it's tightening up before the sheet's all the way on, and the chafe the slack lift can create along the leach of the sail when it's up there and soft enough to not interfere with trim. There's no need for a topping lift if you use your main halyard to support the boom when the sail's down, for example, though you do have to set the boom on the cabintop before you transfer the halyard, obviously.
Nowadays a lot of boats use a 'boom kicker' or another form of rigid vang that provides support from below, negating the need for a topping lift for that purpose. We have such a vang, and 'store' the main halyard at the boom end as extra support when stopped. This also gets the halyard off the mast avoiding halyard slap.
If you want, you could put a quick release on the boom end of your topping lift and store it along your backstay or elsewhere to avoid the problems it poses when sailing.. just remember to reattach before when you drop the sail. As suggested a bungie cord might help here.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)