I think it depends on who exactly you are selling it to and what they want to use it for.
If there is some workgroup that is buying it that is different thing than something that needs buy-in from central IT and top management.
If it is internally facing, I think corporate identity matters less, if it is externally facing, it is critical. If you look at Saleforce.com for instance, the internally facing side looks like Saleforce, but consumer facing forms can be completely customized. Put it this way -- if you send customers to a site that looks like a phishing site that was drawn with a crayon, that is bad.
For internally facing apps I think the best integration feature is single-sign on with the corporate intranet.
