There is a debate in Shas about whether Mitzvos tzrichos kavanah. Regardless, if one does the mitzvah whie having express intentions for another purpose, it is a serious issue.
Here’s the challenge. Especially these days, many people look to escape the realities of life, and this may be far more prevalent than we have believed. If the drinking (within halachic limits, of course) is lishmoh, then one would not be losing himself, barfing on someone’s lawn, getting behind a steering wheel, walking out in traffic, etc. If, however, Purim is just the excuse to “let go”, then we have a major problem at hand. It is not connected to any mitzvah, and is undoubtedly nothing more than a pure and disgusting aveiroh.