Note: I was closely involved in much of the design of HS2, but I speak only for myself, not my former employer.
Without OOC station, there is no HS2.
The numbers of passengers envisaged on HS2 simply cannot be handled by a single London terminal station. By connecting into Crossrail and the GWML at OOC the wider connectivity creates a natural split of passengers between OOC and Euston. Adding further connectivity such as North London Line / West London Line / Central Line to the OOC complex further enhances its utility. Comparison with "What if HS2 ran direct with no intermediate stop" is an illusion, as that could never happen. I did rather offend some members of the GW planning team when we were discussing the need for all GW trains (main and slows) to call at OOC, since I was (and remain) convinced that OOC will grow to far overshadow Paddington in importance for the GWML. I described Paddington as the "necessary turnback sidings for OOC". There was some severe harumphing then, but by the following meeting they were fully on board with the idea.
It now appears that reconstructing Euston is going to take longer than first envisaged, so HS2 will open first to OOC, and only subsequently to Euston. That rather underlines the vital importance of OOC to the whole project.