Basically, exactly what you're trying to sell as an advantage is Skypes main disadvantage: It is not a good internet citizen. It does not use well defined ports. Actually, it hijacks the existing infrastructure in a very impolite way. Again, while you might enjoy this as long as you benefit from it, you have to ask yourself: "What if everyone did that?"

It is a good thing that the owner of a network has control over what's happening. He is legally responsible (in most countries) and he wants to provide a certain user experience for all of his users. Now if you build a hotspot for your customers to surf the web and read their mails, you need to calculate the amount of bandwidth you'll typically need for all of them to surf at a decent speed. Good. Now one of these customers tunnels his filesharing traffic over port 80, uses Skype and does a number of other things the network wasn't designed for... As long as it's one customer, it'll probably go unnoticed (except that there might be legal implications). But if everyone does it, you'll run into troubles.

My experience is that many hotspots don't block SIP for the reason that they don't want VoIP. They simply block everything thats not needed for surfing and mailing. I usually can't chat (IRC), can't do filesharing, can't use instant messaging .... So I don't believe you can't do SIP because they want to block VoIP. If they really, really wanted to block all kinds of VoIP, they'd have to start with Skype simply because it's popular (there are ways to do this, although its more complex than blocking a port).

So the bottom line is that with Skype "works everywhere" translates to "abuses network infrastructure". I wouldn't dare to sell this as a good thing, let alone as an advantage.