IMNSHO if you're going to be climbing mixed stuff, it's a no-brainer. monopoint crampons allow you to climb just like you climb rock. in fact in some ways better 'cause your point doesn't get tired like your toes sometimes can. for me using m/p on vertical ice allows me to climb the steep stuff using the same techniques I use on rock & mixed. I can twist & turn my toes & the like without loosing purchase.
when I got my first m/p crampons, a pair of Moser Grade 8's, my climbing jumped up a grade. your milage may vary. <grin> the only time I use dual points any more is in the mountains. currently I am using the Moser M10's from 2 years back, but they are a bit heavy. I understand that the new ones are lighter. I also like the Trango Hyper Harpoon. their price is definitely right. I got a chance to try the Petzl Dart's last winter & I thought that they climbed OK. the latest versions are easier to fit & get on & off. you can't replace any parts tho, just by new ones when they wear out.
many of the new crampons also allow you to set them up as dual or mono point so you can have your cake & eat it... in addition some let you position the mono point left-to-right if you like.