If a person is impure because of his nocturnal emission. Is he allowed to do mitzvot? Should he still strive to do them or should he keep low key and try to avoid them because of his impure state?
The understanding of the Gemara is that someone who had an uncontrolled emission is that he should not learn Torah since it should be done in a state of kedusha and tahara, in as much as is possible today.
Most Chassidic groups will tell you that the Baal Keri should go to the Mikveh. In Litvishe circles, since anyway we're all tamei, Baal Keri doesn't add to it nor does going to a Mikveh detract from it, and therefore a person should nonetheless learn Torah.
I would venture also, that uninduced nocturnal emission today, is very hard to come by. Most people have impure thoughts, eat the wrong foods, and are at a lower spiritual level. All this contributes to the fact that he is not strictly a Baal Keri.
However, this is related to Torah Study alone. This in no-way justifies not actively performing any other mitzvos - for example, wearing tefillin, saying krias shema b'zman etc.
The din today is that Baal Keri (with regard to tuma today) is a takanas Ezra, and is at best a din D'Rabbanon (most pasken it is Midos Chassidus), and it is not clear that has been accepted in Klal Yisrael as normative halacha. Certainly one should not refrain from learning Torah or do mitzvos because of it. In other words, today the status of a seminal emission, is one that required immersion in a mikveh is only middos Chassidus. In the times of the Beis Hamikdash, a seminal emission is referred to as a Zav and that is a D'oraissa level, and needs ritual purification.