Many startups in YC actually launch during the 3 months they are here. Being successful before entering YC is only true for a small percentage of the companies.
There is no prerequisite to have a "stable tech infrastructure" -- shoot, there's no prerequisite to even have an actual product! -- for admission to YC. pg has made it pretty clear that YC selects founders, not necessarily ideas.
And for those that have a proof-of-concept running on some other provider(s) (AWS/Rackspace/Google), it would be a huge boon to be able to migrate and scale using Azure and the $500k credit.
If what you have is a couple of web servers and a couple of database servers, I would take the one weekend off to learn Azure and write up scripts to migrate data and servers and other configuration.
Would definitely be harder if you have a more complex infrastructure - but in most cases entirely worth it.
This would only benefit YC startups who already use Azure. (Unless the switching cost from AWS/Rackspace/Google is low enough nowadays.)