If we don't engage with Iran, India will. As you can see some Indians here are using this "Irani loyalists" talking point to further a particular agenda. Just yesterday Iran asked China to develop Chabahar port which is good development as far as Pakistan is concerned. We need to reduce India's influence in neighboring countries. And secondly Iran is a rich country (resource wise) and there is economic aspects that cannot be overlooked for sectarian reasons. Pakistan right now is doing great, we have good relations with Arabs (we are also negotiating an FTA with GCC) and on the other hand we have this developing relation with Iran (recently both countries agreed to open state banks in each other countries for trade and also talks on FTA are on going). Pakistan should be a welcome place for everyone, Saudi, Iran, Russia, USA, ... the economic dividends could be huge. I subscribe to Andrew Krybko's idea of CPEC being convergence of ancient civilizations, the new silk route revives linkages b/w those ancient civilization that were connected through the old silk route, Iran, China, Eu, Arab, ....
And secondly if there are security concerns lets say with Afghanistan by not engaging Afghanistan we will be presenting Afghanistan to India on a platter. Same is the case with Iran, if there are security concerns about Iran vis-a-vis India, Iran should be engaged to remedy that.