If you're concerned aboutt the drainage system, I would recommend that you have an additional inspection done by a specialist. Some plumbers are even able to put a camera into the drain so you can see exactly what is going on inside.
Don't rely on the home warranty to take care of the problem; as you noted the coverage is limited, and pre-existing conditions are excluded. For most maintenance issues, and especially for drainage problems, being proactive will cost you much less: i.e. it will cost a lot less to address any issues BEFORE a big problem like a sewage backup occurs.
You will have the most leverage BEFORE closing, so if you're concerned, have an expert evaluate the house's drainage before you move forward with the purchase. Make sure that you check your sales agreement for the wording of your home inspection clause to make sure that you stay within the time frames allowed for inspections and requests to the seller for repair. Your agent, if you have one, should be able to advise you as to what you need to do and when it needs to be done before the home inspection contingency expires.
Also, if you're able to do so financially and if your mortgage company allows it, many buyers prefer to negotiate a credit for the repair so they can hire a contractor themselves to ensure the repair is done properly. If the seller does the repair prior to closing, they may choose the cheapest bidder who may not do the best job. You can ask that money for the repair be put into escrow at the time of closing.
**If the plumbing inspection finds that the main sewer lateral (i.e. the connection from the house to the street)needs to be replaced, make sure that you ask for any landscaping that needs to be dug up for the repair to be restored at the seller's expense (or have sufficient funds put in escrow for this). Otherwise, you'll end up with a dug-up front lawn or torn-up driveway.
Good luck with your transaction; if the house is otherwise is good condition, don't let small, easily fixed issues scare you!!
The inspection contingency lets you do any reasonable (i.e., non-destructive, at buyer expense) inspections within the time period. The general home inspector can identify defects, but usually doesn't have professional level expertise in every system, and can't give you detailed repair estimates. So it's common practice to call in "specialists" when the home inspector finds something and you're still concerned.