We're doing a 2.5 week trip in Jan/Feb and we're staying on-site. I've done this before and this what I can say helped me personally to make it feel both worth it and comfortable:
1.) As others have mentioned, there's less pressure to be in the park from open to close. You can very easily go to the park in the morning, take a nice break in the afternoon when it's the hottest (and it will be very hot in the middle of the day), then head back to the park in the late afternoon/early evening. Whenever I do this, I get tons of rides in, I feel rested for being in the parks at night, and I'm just happier overall. If I stayed offsite, I would not take that break in the middle of the day.
2.) Doing a split stay helps break up the monotony of staying at a hotel vs a condo/apt/villa type of place. Changing the resort makes it feel exciting all over again, like you're going on a second vacation! We are doing a split stay with Yacht Club & Poly. Both resorts are great places to relax at and will put you in close proximity to the park. Especially if you choose BC. From there you can walk to HS or EP. You can also take a boat or the Skyliner to HS. But Poly also has some good options for getting to MK (boat, walk, or monorail). You can't go wrong with either.
3.) It's easier for us to split up during the day. We are going for 2.5 weeks next year so that my husband will have lots of rest time. He will be in the parks for 2 days, then take a day off to rest at the resort. Staying onsite makes it easier for us to split up. If we stayed offsite, I would be leaving him without a car while I went to the parks. But with us staying onsite, I can head to the parks if he decides to join me later in the day, he can very easily.
Honestly, I've never gotten bored with staying onsite for long trips. But I spend a lot of time at the parks or enjoying the resort pools. I also don't like cooking on vacations, so having a kitchen is not a necessity for me. I'd say at most, we eat a quick breakfast in the room before heading out. Also, we don't have kids, so when we go to the pools, if it's a little too loud for us to relax, we just bring waterproof headphones.
Anyway, I know how we do things is a little different. But long story short, I find staying onsite helps keep me in that magical bubble. I'm a
Disneyland (in California) local and I find that driving to the parks when I go and not staying onsite always makes me miss the WDW bubble. I'd much prefer to just be able to wake up in the resort onsite and either walk or use resort transportation to get to the parks.