We'll have 13 days, and since the 10 day tickets are relatively cheap (compared to, say, 5 day) it doesn't make sense for us to hop. We are happy to only spend half days in the parks, or leave at lunch time and go back to the same park in the evening.
For a shorter trip, or a very busy time when you might want the flexibility of leaving a super-busy park, or if you want to keep going to your favourite rides without spending all day in one park, or if you love commando touring .... then hoppers are a must.
But as it was said, you can add the hopper option later, it's a fixed price. So if you want to save money, try not hopping. If you find that isn't working out and you feel the need to hop, go to Guest Services and have it added.