Well, all other things held equal, most customers would prefer to buy from an existing vendor (one throat to choke and all). Given the nature of most RFPs for this kind of software it's hard to win on value since the requirements are laid out in advance; so you almost have to win on price. The danger of winning one of these contracts is that SAP/Oracle/IBM all know how difficult their ERP systems are to integrate with, and if you can under-price them, you're likely underestimating that cost.

Basically, there's a lot that can go wrong and you need to have a certain scale before you even attempt to go after the big whales. You shouldn't be trying to work with them while building out the basic version of your product.