Sean - that is an excellent question. Do NOT sign a P&S without talking with a real estate (title) attorney first who can explain this to you in detail.
My layman's understanding is that marketable title means no cloud or blemishes on the title. It is possible to have a blemish on the title but a title company will still insure it. That would be considered non-marketable, but insurable title. Again, talk to an attorney to get a more complete explanation.
My opinion as a real estate agent is that non-marketable, but insurable title does affect the value of the property. Some buyers will accept this title and some will not. Also IMO, the title issue should be disclosed in the Sellers Disclosure. Non-marketable but insurable title does not make it impossible to sell a property but perhaps a bit more difficult. The price should reflect that.
Again, get legal representation before buying a property with title issues. And ask your buyer agent their opinion of the impact on the value.