Do I really need a chimney liner?

[Jacob Elbe]

I'm buying a duplex and the home inspector brought up a concern about the furnaces and hot water heaters venting into the old chimney on the house.  There are no fireplaces.  I've never heard of this before, so am hoping some of you can tell me if this is really a big deal or not.  He explained that over time the gas going into the chimney can erode and deteriorate the inside of the chimney and cause a lot of damage.  He also said another concern is these gases leaking into the house.  He recommended having a chimney expert come out and give a quote on putting in a stainless steel liner.

Should I ask for the seller to have a liner installed?  Or just use it as a negotiating point because it's not really an issue?  

Is this a normal issue in a house built in the 1930's?  Has anyone actually had a problem caused by the lack of a chimney liner?

Thanks!