They will make you slow and stiff, you won't be able to take a punch and you won't be able to flex your elbow to be able to form a guard and defend anymore as your biceps will get massive if you even look at a weight. Weights = slow and unflexible. Look at NBA + NFL players, sprinters and Olympic weightlifters for proof, those guys have all been ruined by lifting weights in their training. Just think about how good they could be if they didn't lift weights, such a shame.
Good, but it depends what kind of program you are on There are sports specific lifting routines you can put yourself on Certain exercises aren't designed or useful for certain sports A Lightweight boxer and an NFL guard don't require the same needs
Definitely useful if done correctly. Remember you're training to be a boxer not a bodybuilder so there's no use in doing hundreds of bicep curls. Olympic style lifts (range will vary depending on what you'd like to achieve i.e. speed, strength, power etc). There are a few strength & conditioning articles for boxing here: www.mdmboxing.com/training/
weights can be used for different reasons Strength training Bodybuilding Functional training I would train Functional for boxing Squats,Deadlifts,chin ups etc and sprints