In short, yes, a live chat is considered to be a customer benefit.
I am not sure what you mean by it adding "weight" to your website though as there are plenty quick and light solutions on the market.
My suggestion, however, is not to offer a live chat if you do not know what kind of chat it is going to be (sales/support/etc) OR if you don't have the resources to make sure someone is always there to respond. Nothing will spoil the first impression and the reputation of your company as a dead chat, especially during work hours. It can get worse if clients experience it repeatedly.
Another point is to understand how that live chat will work within your system. How will it be integrated into your CRM? Will it even be integrated? Imagine how many people might opt to contact you with a sales question over a chat rather than email. Are you ready to let all these potential leads just go, without an opportunity to follow up? If not, then you have to plan for how these will be recorded into your system.