I agree with Mike P. and Tom, I have had more ticks on me from non hiking related activities than on. And when I have gotten ticks on me while hiking it was while wearing shorts, but I was able to see and feel them long before they got to start burrowing. I assume even if you are wearing pants you glance down often enough to notice if something may be crawling on you.
Either way, I bought a couple pairs of Nylon zip-offs when I first started hiking like 4 or 5 years ago, and I still wear them. Occasionally in the summer I even go for (GASP!) plain old cotton shorts. I sweat way too much that synthetics, when damp from my sweat, feel annoying to me and uncomfortable.
Zip-offs= the best of both worlds.
Brian