| There are two problems with sealants. They won't do a think if you have a slice in your sidewall, and when used you essentially have to throw away the tire, as they change its balance and weight and the way it handles heat.
It might make sense to carry around a tire patch kit, so you can quickly plug a hole the same way a tire shop would do it, and can continue to use your tire normally. About $5.
Still not a solution for all cases like a spare, but at least it doesn't destroy your tire like tire goo and is cheaper too. | 
