Opinions concerning spark plugs

[mnc-i]

Reference routine maintenance:

My 1999 Boxster is my daily driver and I focus on properly maintaining it. I now have well over 228,000 miles. I change my oil & filter every 10,000 miles and I change my sparkplugs every 30,000 miles (During the last two sparkplug changes I've used Bosch Platinum +4 plugs).

My question is this: Do I really need to change sparkplugs every 30,000 miles?

I am interested in your opinions on sparkplugs. I know early model 986s have different maintenance intervals than later model 986s.

I purchased it 5 years ago last September with 104,000 miles. In the past five years I have replaced the water pump twice and the AOS about three times (it failed again two weeks ago). I don't abuse it and I seldom rev it past 4500 rpm.

Thanks
MNC-I