The knee-kerk reaction against this event (and marketing/pr in general) on News.YC is pretty daft, IMO. There are some (very social) services that require "critical mass" to be engaging. The spike of TC50 MIGHT just push a service like that into having a self-sustaining/growing user population. Example: What Twitter experienced at SXSW.

And for anything inherantly, viral-- an event like this is a big bump... It's compound interest with a larger initial principal. If you have a viral service that starts with 2 users and I have a viral service that starts with the 10k I'll get at TC50 and we both have the same viral co-efficient, who wins?

And for any company that is fundraising, the visibility/credibility provided by this event is HUGE.

And for any company that is actively looking for partners/bizdev deals, the visibility/credibility provided by this event is big.

Is the value big enough to be worth shifting your launch date dramatically? Probably not. But if your launch date is in the neighborhood, trying to get it done at TC50 (or a similar event) is a low-cost/high-value proposition. Hell, even DEMO is low cost compared to most other PR/marketing spending you could do.