Startups are really fucking hard. You have to have a serious belief and commitment to build a billion dollar business (which is what YC is investing in.) If you're unsure, that's totally fine, but you need to be the first to believe in your company and get used to being the only ones who believe in your company, in order to make it through the hard times.
I have bias to disclose - I'm in the 2nd category, and my ideas and projects are probably very different, with a very different target market, than me 15 years ago (or someone else 15 years younger than me right now). I'm also not seriously pursuing anything at the moment, so this is hypothetical: Not only do I have a roof to keep over my family's head, I also (no offense intended here) probably have a higher opportunity cost as a barrier as well. All in on an idea for me has hard and soft costs that are very different from someone just starting out. It's not to say someone similar to me should expect to throw a few hours every two weeks at a project and merit serious consideration, but putting 25% of time and energy for some is a BFD that I think passes the commitment threshold in certain instances.
Whether it's YC, a startup, or your role in a mature business of any size, you must approach ideas through various lenses beyond your own. Please take this all constructively, I'm not trying to say I'm better because I'm older or anything like that - my point is that there are broader considerations that make varying degrees of commitment to an idea more similar than you'd think.
(/s in case you need it. If you still don't get it I am making fun of your position because it's naive and in my experience wrong. Sorry. Privilege comes in many forms and people don't tend to talk about them.)
That you are smart enough not to weaken your negotiation position if you don't actually have to?
This isn't a game theory experiment. It's not unreasonable for investors to want you to be full-time invested (if you can) before putting in money. Also, some people may want to run their company until it is profitable enough that the company can pay them a full-time salary.
Now if you're YC and you see 2 identical companies and one set of founders is full-time and the other set of founders are part-time, it's safe to say that the full time founders will be given preference because they are more committed, etc.
On the flip side if you had phenomenal founders/company (at an extreme think if Mark Zuckerberg started a new company and applied to YC) being part time is not a big deal.
Unfortunately for you you don't have visibility into your competition for this batch, but you can use previous batch companies as a rough guide.