Should mobile PCs have a thumb keyboard?

As more healthcare professionals use smartphones (BlackBerry, Treo, etc.) that have a thumb keyboard, they're getting fairly proficient at thumb typing. You don't need to have a T-Mobile Sidekick to know how to use a thumb keyboard.

Does it make sense to include thumb keyboards on more mobile devices such as ultra-mobile PCs (UMPC)? The OQO model 02 had a thumb keyboard. The Sony Vaio UX series also had a thumb keyboard. Samsung added a thumb keyboard to the Q1 Ultra UMPC series when the original Q1 did not have a thumb keyboard.

Does it make sense to have a thumb keyboard on smaller slate tablet PCs? If we follow the Samsung model, we see this:

• Original Q1 (slate only, no thumb keyboard)
• Q1 Ultra (slate plus thumb keyboard)
• Q1 EX (slate only, no thumb keyboard)

Does this reveal that the thumb keyboard on an ultra-mobile slate tablet PC was a flop? What about sliders? The OQO and Sony UX both included a thumb keyboard. Even the Nokia N810 MID has a thumb keyboard. However, the Viliv S5 does not. Does that make sense?