If you are sued and you hire an attorney to represent you, who does the talking in the courtroom? Would it make sense for you to call the plaintiff's attorney or even the plaintiff directly to discuss the case? The answer is obviously no.
It’s the same in any negotiation in which an agent or intermediary is hired by a principal. You can't successfully negotiate if there are multiple people contacting the other side. Not everyone would have the same information when needed.
So it's important that your tenant representative (corporate real estate advisor) be the single point of contact for all buildings under consideration - even with your current landlord. Otherwise, the tenant rep can’t possibly provide you with objective advice because he won’t have all the details of your conversations.
If you are considering a move of your office from Frisco to Plano, there will be well over 100 buildings to choose from. It wouldn't make sense for your tenant rep to consider only 99 of those buildings while you dealt with the last one. When the tenant rep makes a recommendation, he will do so without knowing what the 100th building is offering.
You hire a tenant rep for many reasons one of which is to use his expertise to negotiate on your behalf. He may choose to present information in a specific order or at specific times to improve the tenant's position. If the tenant continues talking to the landlord and leasing agent, the tenant rep can't manage the information provided or the timing of its delivery and that may compromise the negotiations and hurt the tenant.
Besides, the tenant has more important things to do – like running a business.