They're fragile but I've found them to be a great firepower boost/distraction. Outflanking Scout Sentinels can cause yahoo in the opponent's lines and with cover can be suprisingly hard to kill. Equip them with Multilasers or Autocannons and HK Missiles and they can wreck some things. Then even if they're rage killed the next turn at least that's one less thing shooting at your main army. I'm not quite as big a fan of Armored Sentinels because they're still fragile but they are nice to run with a mobile army and/or flank your tanks to keep Deep Striking goons away.
For laughs I made an Unbound army list that was ALL Sentinels. I only have 16 currently but I really want to see an army that can potentially fire 37 HK Missiles plus Lascannons, Autocannons, and Multilasers on the first turn.