Dec/Jan/Feb are not pool weather.
We stayed off-site once (just the adults), several times at the Polynesian. We had two tweens and a toddler one time, later we went with just the youngest who was about 7 or 8. The Polynesian is definitely pricy but we loved that it had a little shuttle boat that went just from there to the Grand Floridian to the Magic Kingdom. Zero crowds for that. We could walk out of our room, walk down the hotel path, and be right at the monorail to get to Epcot. We could easily go in the morning and have lunch, then come back to the hotel to chill out. We put the little one down for a nap and one adult took the older kids to the pool or one parent took them to the "big" rides that the baby couldn't go on. We also got a babysitter through Disney and had adult/tween time a couple of times, real dinners, etc.
There were buses to the other parks. In general, I think Disney handles transportation very well, moving large crowds with efficiency and a bit of entertainment.
We did not do the meal plan but had a fridge in the room (perfect for the littlest one) and we took a small cooler with us to the parks so we didn't have to wait or pay for restaurants/snack bars.
You have to weigh the cost of a rental car, parking, commuting time, etc. against the convenience of being on site. I know a lot of people who have rented a house off site, especially with a pool, so they can have down time at "home" and cook their own easy meals (grilled stuff, take out pizza) and not be so focused on the parks. That's more affordable if you have a large family or two families.
The one time my husband and I stayed off site, it was okay - a no frills chain hotel with a free shuttle to the park, and we were comfortable staying all day and getting all our meals at Disney. But we only did that for 2 nights, I think.
As always, it's weighing the cost vs. the convenience.