Under what circumstance would setting MaxDop to anything other than 1 cause an issue?

we are having continued discussions with our vendor whom is insistent on setting MaxDop to 1 we have performed internal testing and see a general 20% performance improvement by setting this to 4 with cost threshold to 50 we suspect they may be other motives of insisting setting of 1. I'm unsure what they maybe however. ** EDIT:: to make the question more specific, is there a scenario where by setting maxdop to anything other than 1 cause errors to occur** - Microsoft Guidance https://support.microsoft.com/en-us/kb/2806535 - relevant Paul Randall post http://www.sqlskills.com/blogs/paul/maxdop-configuration-survey-results/ - Brent Ozar on CX packet waits http://www.brentozar.com/archive/2013/08/what-is-the-cxpacket-wait-type-and-how-do-you-reduce-it/