I think this sums things up quite well: LINK
When you plug a tyre from the outside, you do not know if the tyre has damage on the inside. My wife drove on a flat brand new tyre, one day old. It looked perfect once inflated but when the fitter took it off the rim the inside of the tyre wall was shredded. My daughter did the same thing recently.
You don't know how well the plug is fitted. If the tyre is off the rim you can see the inside of the plug. The plugs garages use also have bigger heads, because they are fitted from the inside. The head of the plug cannot go through the hole.
If you plug a tyre, a garage won't fix it.
Having a spare wheel should be mandatory.
Bikers probably aren't a good example to cite as everyone knows they're crazy anyway! ;0)