SMS and email are different. SMS costs $ per message. Email does not. In the Facebook UI, if you add a SMS user to an email thread, he will receive numerous SMSes from email users who don't realize that they're racking up his bill.
Wait a minute, is that true? Do you pay for receiving text messages in the US? That does seem a little crazy from a European perspective.
It's absolutely true - foregoing a texting plan in the US saves a few dollars every month. This is popular enough that it's impolite to assume that all recipients will appreciate text messages.
When I signed an agreement in college, I texted so infrequently that I saved money by paying for individual messages. Now that I can afford it, I still pay per message as an excuse - I can ask people to call or email without sounding rude.
Most heavy users of SMS (aka teens) have messaging plans that are either unlimited or very high numbers, like 1500 per month. Usually $15-20 per month, which seems high but they generally replace actual phone talking so it works out.
Also, it's funny that Apple is being brought up in this context. The iPhone treats SMS messages just like chat messages (using the iChat UI), which is exactly the "problem" being described in the article of mixing messaging types carelessly.
1500 SMS/month for $20 sounds like daylight robbery to me.
Here in Pakistan, we get about 8000 SMS/month for $1. So it's not unreasonable to think that SMS would take off really well.
If you are a heavy texter, there are much better plans on other carriers. For instance, Virgin Mobile has unlimited text and data and 300 minutes for $25 a month.
SMS and iChat may have the same interface, but they are not mixed in a single client. In fact, Apple does not provide an IM client. The central complaint in this article is that the same message was being communicated over email and SMS, without the sender being able to distinguish between the two in any way. I did not read this article as complaining that the interfaces for SMS and email were too similar; I read it is complaining that they were in the same client. The only complaint about the interface I saw was that it grouped all messages together, independent of the subject.
[Edited to clarify which part of the parent I was responding to]
European carriers charge each other to receive SMS as well, they just don't customarily pass it on to the consumer. The mobile network "Three" used to have a program where they'd pass on the money received to consumers, and it became a scam, popular in Italy, to send yourself hundreds of SMS (from "free SMS" websites) to get free money.
Technically you can get some nominal charge per text, but most people have an 'unlimited texts' plan. It's like buying a phone and not having minutes now days
Wait a minute, is that true? Do you pay for receiving text messages in the US? That does seem a little crazy from a European perspective.