A now deceased priest in our diocese was once loudly BLASTED by a parishioner for not telling people to join hand during the Our Father his first weekend in the parish.  "You've set this parish back 25 years in its spirituality!!"  The priest responded, "Actually, ma'am, your yelling, your anger and your lack of charity has done more to accomplish that."  As for the unity rationale, St. Paul tells us that what unites us to Christ and each other in the Body of Christ is partaking of the one bread and one cup -- in other words, Communion.  My guess is the hand-holding gesture began in churches where Communion is celebrated infrequently.