David H. Lipman wrote on 2016/01/04:

> There have been many discussions about bugs in MBAM v2.x and whether we 
> should "switch" from v1.75 to the newer v2.20 product.
> Although Malwarebytes has not yet officially retired v1.75, Malwarebytes is 
> now applying signatures for "file level scanning" that are v2.x dependent. 
> That means v1.75 will now fail to detect many files in which the new v2.x 
> signature constructs are applied to. This can create a False Negative 
> situation where v1.75 will fail detect malware on a PC not because they 
> aren't yet detected but because the that version will not detect them. At 
> this time this also is a problem with Virus Total. They are using a v1.75 
> based engine so files that are detected with v2.20 will not be shown to be 
> detected on Virus Total as well.
> 
> Malwarebytes has been silent about this change and that's not good. It is my 
> opinion that applying signatures for "file level scanning" that are v2.x 
> dependent should NOT have been implemented until v 1.75 was officially 
> retired.
> 
> Therefore it is my "strong" recommendation that all users, using the free or 
> paid-for version of MBAM v1.75, must now move to v2.20.
> 
> MBAM v1.75 is no longer reliable, viable or "safe" to use.

The prior problems with v2 of MBAM had to do with its on-access
(real-time) scanner.  The free version does not include the on-access
scanner so it did not exhibit the problems.  There was no point in
sticking with the old free version.  
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