Bilateralrope said:
Look at this the other way - the Techmarine can make a melee attack as a Reaction while still firing away at another target with a Full Auto Burst from his bolter . That's pretty useful.
Except that basic weapons can't be used in melee, so you should be talking about a semi-auto attack. Still useful, but not as much.
OK, engaged characters can only make melee attacks. So you could try to use the Reaction for an attack while using the Disengage action to move to somewhere to shoot from.
And now, for total cheese:
Blood Angels Techmarine uses Reaction to attack a melee opponent with his Servo-arm. Then, since he's a Blood Angel and has taken Acrobatics and Assassin Strike, he flips out of melee as a Free action. Now he uses his bolter at point blank range to put a Full Auto Burst into his target.