This is always a hot topic in my family as we have several members who own DVC and we like to stay offsite. We travel as a bigger group every other year (just came back from another trip to WDW with extended family on Sunday) and I have some observations and experiences made by me and some of my DVC family, who have either stayed with us or visited our resort.
First, we always have way more room in our offsite condos. We like to stay at the Marriott Sabal Palms and you can't beat the size of the 2 bedroom, 2 bathroom condos with full kitchen, washer and dryer, living room and dining room. This trip, half our family stayed in a lock off in BLT and it was very small in comparison. We initially were going to stay with them as well, but extended our dates and stayed the entire time offsite.
The pools at Disney resorts always seem to be super crowded.
Many of our DVC family like to drive to the parks, rather than rely on Disney transportation. Just this trip alone, some of them got delayed as much as an hour taking the monorail to Epcot (twice!) where we were waiting for them. The exception to this is when they stay at BLT or the Contemporary and can walk easily to MK, which is the most pain to get in and out of. Even if our DVC family flies to FL, they will rent a car because they hate the buses.
Cost: this is a biggie and I don't know how other DVC owners do it, but the dues/fees alone can be a burden. This is a public message board so I won't put too much personal info here, but one of our family members bought in last year and their adult children all help them pay the dues, which is a hefty amount. The downside for them is that they all stay together and some have to resort to sleeping on pull-outs when they go. That may not seem to be a big deal, but for the cost of one of those adult children's share of the dues/fees over the year, they could stay at an offsite resort for two full weeks and sleep in a bed. But...and this goes with that idea of pressure from staunch on-siters... (some of which are our family members), "you can't stay in Disney without staying on-site". So they bought in when they probably shouldn't have. I also have a friend who maxed out a credit card a few years ago to "do Disney" and is still paying her trip off.
I am grateful for options. If people want to stay on-site, go for it. Same if you want to stay off-site. I'm sharing my experience and I don't judge if someone wants to stay on or off site. I just don't like the pressure (which I've seen and experienced first hand) or the other arguments in favor of onsite (transportation, who needs a kitchen?, you're only sleeping in the room so you don't need more than a bed, you won't get any good fastpasses at 30 days). Everyone has their own priorities for vacation and there's no wrong way to do Disney. We had another great Disney vacation staying off-site and are already planning our next one. The amount of money we save allows us to do more, like the Star Wars Dessert Party, ROL dining package, Rose and Crown for Illuminations. That's the way we like to vacation at Disney and we enjoy going back to a quiet, spacious resort to swim in a pool that is never crowded. That relaxing down-time is important to us.