This sounds like something which is obviously true; but I'm not so sure: In particular, it seems to me that an investor's involvement is likely to depend as much or more on their investment as a fraction of their assets as it does on the absolute size of their investment. If I invested $100k in a startup, it would be because I really believed in it, and I would help them in any way I could; but if Warren Buffet invested $100k in a company he would probably forget about it before lunch.
To the extent that "party rounds" involve large investors throwing their spare change into a pot, I agree that they would certainly not be productive; but if a party round allows small investors who would provide advice and assistance disproportionate to their assets to get involved, it seems like it could only be a net positive.
After all, nobody expects that an investment of only $20k means that YC won't provide much help.