Services work using an Engagement Letter vs. a formal Master Services Agreement

I recently started work for a services agency and the past Controller/CFO didn't ever push the issue with having a formal contract in place.  The thought was so long as the money was paid and the payment terms were somewhat outlined in the Engagement Letter, then all would be fine.  Company has operated fine under this mode for 10+ years.  I prefer a little more formality in my engagements and have started pushing for formal agreements.  Needless to say, there is some push back, not to mention what to do on future engagements with long-time customers that are not used to formal paperwork.  

Question for the group...have any of you had similar transitions?  Advice on how hard to push?  The owners (S-Corp) don't seem to have a strong preference, but are supportive as long as it does not disrupt business.  Does the marketing/services world work more in Engagement Letters than formal contracts?   Past years write offs have not been terrible.  

Appreciate the feedback in advance.  

Mark