It would seem you have already made up your mind not to sign it by your statement above, if you plan on staying with the company I see no reason not to sign, but if you do plan to leave it could be sooner then later if you refuse.I understand his point, but Im not out to steal business and wont sign it. Is this common for an employer to ask. Its not like they pay enough to be exclusive.
Kinda' a hard place to be in, you made them look bad (normal print staff) and good (with the customers) at the same time, I would think your employer has noticed that which is the reason he wants you to sign.....ultimately no one can make the decision for you.
Hope this helps.