[madamelic]

And building on barebones keeps all options open.

If you go big and realize an IaaS solution is right, great, you are ready to go on to IaaS with no trouble

If you IaaS from the beginning, you've cornered yourself and have to build your way out.

[chii]

How are these scenarios any different from each other?

You are not ready to go IaaS with no trouble if you've already built your own. It's just as hard to do that as going from IaaS to your own custom infrastructure!