If you're not going to use some of the features of Ultimate -- such as Aero, Media Center, BitLocker, XP Mode, joining a domain -- it's probably a waste of money. You might not even have hardware sufficient to run Aero or XP Mode, and you'd need a tuner for Media Center to be very useful.
On the other hand, BitLocker in particular is useful for portables, from a security perspective. It can basically encrypt your hard drive contents. But, on the other other hand, you could do that anyway with a free tool like TrueCrypt. :-)
Save your money and buy the edition that's right for you, after analyzing your needs. Even if you need one or two of the features listed above, there may be a more appropriate version than Ultimate. The following page at Microsoft will help you choose editions: Which one is right for you?