Other than speed, is there a reason that a 10W iPad charger shouldn't charge a new MacBook Pro 2016? Are there any reasons not to use a 10w ipad charger with a USB->USB-C cable to charge a new MacBook Pro 2016? Have the one charger that came with MacBook but would like to be able to plug in at office, home on road, etc. with other chargers. Thx.
A 10W iPad charger should in theory work and you would be correct that it would be a slow charge.   Well, to be accurate, it would be a horribly slow charge - you are getting a charger that is 1/6th (17%) of what is required (61W).

Will there be a problem?  No.

Will it charge?  Slowly.  Molasses in January slowly. From Apple's web site:


  Your Mac will charge from USB-C power adapters not manufactured by
  Apple if they adhere to the USB Power Delivery specification.


Assuming you are using a genuine Apple iPad charger, it should charge since Apple charges adhere to the spec.