In my experience, most of the "Extended Service Plans" sold by Big Chain Stores have too much fine-print. Everything they give you in 'the plan'... they take away in fine-print. Most require you to have the ring inspected every 6 months and if you miss an inspection they don't have to cover it. Also, in the case of lost or missing stones, many will cover the labor to replace the stone but NOT the cost of the stone itself.
I recommend finding a well established private jewelry store who considers an "extended service plan" the same as customer service and it's FREE with any engagement ring purchase. Things like Cleaning, Maintenance, Stone Tightening, and Accent Stone Replacement should be covered for 5 years free of charge. That's not to say you can crush it in a car door and expect them to fix it free, but if an accent stone falls out during normal wear - that should be covered.
As far as protecting a larger center diamond, that's where insurance kicks in - and that will protect against loss or theft as well. If the ring is well-made, you aren't going to lose the center diamond with normal wear... generally that's from something traumatic happening to the ring. A good jewelry store should help you with insurance and the cost is minimal - usually around $13 per $1,000 per year. So if the appraised value is $5,000, then the cost of insurance would be around $65 per year (less with a deductible). In summary, if a store is trying to SELL you their customer service, read the FINE PRINT!