You really don't need adjustable poles with a tarp--you can use a clove hitch to fasten the guyline to the pole and put the knot at whatever height you need.

The exception is if you're like me--the pole length I need for hiking is shorter than what's needed for my shelter! In my case, adjustable poles are absolutely needed. Also, if you're going to scramble, you want to be able to shorten your poles and fasten them to your pack so your hands are free. In some cases (several people in the vehicle) I've also had to shorten them for traveling to/from the trailhead.