For the past five years I have carried a 23mm Gatorskin folder or a 28mm Rubino Pro both of which weigh just under 250g. I dont carry a spare on my 26" wheel bike, just because I havent bought a suitable folding tyre and I already have too many tyres for the bike.
A spare tyre isnt a necessity unless you are against the clock and even then the chances are slight enough that it is worth the risk of not having one, you can phone a friend, hitch a lift, borrow off the cyclist who does have one or just give up on the event.
At 250g it doesnt need an awful
lot of justification to carry, its main purpose is to ensure that you can carry on riding at least as far as a shop and until such a shop will be open. Tyres always blow out at 6.15pm on a Saturday up the Cambrian mountains, it is the law.
That 250g can easily be more than offset by riding with much lighter tyres on your wheels but are not armoured against sidewall attacks. So instead of two Schwalbe Marathons at 800g each you ride with two Kojaks, Hypers or similar at 400g each and carry a spare in case of the more likely sidewall damage, so not only are you carrying 200-600g less weight you have drastically reduced rotating weight. This only works out if you do
make a change from sturdy tyres to lighter ones.
I have only used mine once, 1 am in Calais, I rolled off the Ferry with a twisted rear wheel, after fettling it still wouldnt run with the 37mm tyre but would with the 28mm tyre, the spare was good enough that I rode all the way to Bordeaux with it and there I had time and facilities to repair the wheel in comfort. My schedule was too tight to take time out dealing with bike shops.
I am happy carrying my spare tyre but for the five years before I was happy not carrying a spare tyre. Blow outs have been patched with a boot and a Schwalbe Marathon that started a bulge in Bath developed to the size of my fist by the time I got home but I had mudguard clearance for it and had time and opportunities to visit shops along the way.
So lay out all your kit and think is that spare tyre better
use of 250g than anything else that I am taking? and pick which to take.