This sleeping bag liner is the perfect solution to protecting your sleeping bag while still keeping things lightweight. Not only is it lightweight it is also warm.
Instead of getting the inside of your down sleeping bag dirty get your liner dirty. Down sleeping bags shouldn't be washed as it reduces their effectiveness. This is because the down lumps together when wet. Breaking up the down can take a long time. In more cases than not, after washing your bag the down will lose it's effectiveness and your bag will feel colder.
In other words spending a little more on a sleeping bag liner could save you money long term.
The other great think about sleeping liners is they can be used on their own as sleeping bags. This is particularly using on hot summer nights or when staying in overcrowded huts which can get a little too warm.