I've traveled WDW with and without PH - both have there benefits depending on the composition of the traveling party, time of year, and length of visit. And yep - I have bought a PH on the last day of a trip JUST to get into AK for 3 hours because I just ultimately didn't have it in me to skip the park that trip as originally planned. As Accident previously posts - it's really, really easy to change your mind during the trip. It's quick and easy to add them at a park window - especially in the afternoon.
In YOUR case - I strongly recommend against them. I don't think for a trip your length and the fact you have a toddler - they are necessary at all. It takes time to hop between parks. Back to the front of the park, wait for the bus, ride bus, get back into the next park. Best case scenario - I think you lose an hour of park time. With a toddler who only has so many waking hours in a day - that's a waste, IMO. You'll have a better trip if you have a reasonably-rested kiddo so you aren't dealing with the "I'm tired" tantrums.
I don't think AK is necessarily a half day park for you. There are three great shows, the safari ride, character meet and greats, the animal trails, the play area in DinoLand and the train ride out to the area with the petting zoo. To do those at a leisurely pace with diaper changes, snack breaks, freedom to be distracted with a toddler - and that's a RD to dinner time day.
Go with base tickets for now - plan out your days - and then - realize you can change things up when you are there based on how the trip is going.
While I think everyone can function without PH, for families with active older kids/teenagers or just a group of nightowls - I totally understand why those groups value PH and I'm more likely to suggest the option to traveling parties like that.