Baby sleeping bag

[chickers]

Am on the verge of buying on ebay a vango baby sleeping bag. Do I really need one??
My original plan was to layer up my 10 month old ie: vest, babygro,extra socks over baby gro, grobag and then light duvet and put her in a travel cot to sleep in the tent at night.
If she was unsettled or cold in the night I was going to snuggle her in with me and DH but talking to people they all recommend I get her a sleeping bag. Anyone got any thoughts?

I put DD (18 months) in vest, pjs, fleecy all-in-one, grobag and blanket if she needed it and she was fine. Oh also a sheepskin underneath her, on the travel cot. In fact, she loves camping so gets really excited when she sees the cot come out!

Thanks will just layer her up I think, good tip on sheepskin will dig mine out.