I have a Hennessy Hammock in my BOB but...

Lean-tos are nice if you can have a fire for it to reflect heat from but that is about their only advantage. If you are in an area where fire is not allowed or not practical, a lean-to loses that advantage.

Small enclosed tents or hammocks are much better protection from weather and insects and can be just as light and compact. Slightly larger tents give you room to move around a bit or work on equipment while protected from the elements. Most packable tents have a detachable rain fly that can be rigged as a lean-to, although a bit small for that purpose. You could always make an over-sized rain-fly for a tent to increase your shelter's capabilities and versatility.

Note to self: Get over-sized rain-fly for hammock.