You should ask a EA/CPA for a proper tax advice, but to the best of my understanding it may be true.
Transportation expenses include the ordinary and necessary costs of
all of the following.
Getting from one workplace to another in the course of your business or profession when you are traveling within the city or
general area that is your tax home. Tax home is defined in chapter 1.
Visiting clients or customers.
Going to a business meeting away from your regular workplace.
Getting from your home to a temporary workplace when you have one or more regular places of work. These temporary workplaces can be
either within the area of your tax home or outside that area.
You can claim mileage on travel that is not a regular commute. If you work at your clients' - that's a regular commute. What's not is, for example, going to a meeting elsewhere while you generally work for that client in a specific office.
Consider you have this situation:
You live in San Jose, CA.
Your client is in Mountain View, CA.
You work either at home (San Jose, non-deductible
since not exclusively used for business), and once
or twice a week you come to Mountain View
(non-deductible as commute).
You're required to go to the Moscone center for a
meeting with the clients' client on behalf of your
client for a day.
You can deduct the mileage from Mountain View to San Francisco.
If, on the other hand you only go to clients' sites for occasional meetings, and regularly only work from home, then you can deduct the trips to your clients even though you can't get the deduction for the home office. The fact that its non-deductible doesn't make it less of an office.
This is my understanding, but I may be wrong. Consult with a tax professional.