I prefer a silk liner to a sleeping bag - if sleeping indoors and not camping out. Dorms can get very warm at night with all those bodies pumping out heat as they metabolize the evening meal. If cold, you can sleep in your clothes. On my walks, the places I stayed at nearly always had extra bedding if I needed it.
My liner weighs 18 grams and rolls up to the size of a closed fist. A lightweight sleeping bag is much bulkier and would weigh roughly 800 grams. I prefer to travel as light as possible, with only a small pack so the liner is a better choice for me.
Thin liners can be fragile, so if you thrash about in the night, they can rip.
You can buy heavier liners that might be a good compromise choice, especially if you plan to walk in colder weathr. I have a Thermolite Reactor liner that weighs only 240 grams and rolls up into a cylinder 15 cm long and 10cm diameter when packed in its little case.
Hope this is helpful.
Bob M