There are plenty of reasons not to speak in tongue, I guess. Here are some reasons TO speak in tongues.
They are a gift from God. We want everything God has for us.
Paul, who wrote a vast majority of the NT Epistles, spoke in tongues quite often - and encouraged followers of Jesus to do the same.
Tongues create a portal through which the activities of God can happen. Whether initiating your gift in worship, song, prayer ministry, or intercession (hard core prayer) the gift of tongues provides exterior space for stuff to happen, and interior clarity of mind for the one speaking in tongues. One could say that participating in the gift of tongues invokes God into the place.
A person who speaks in tongues will experience a shift in their devotional life. Empowerment, increased faith, accuracy in prayer, and increased anointing all happen when a person unites their glottal with the Holy Spirit residing within.
When people pray in tongues, the devil goes crazy. Sometimes he fights. Most of the time he flees. That's a good thing. This gift is awesome in times of spiritual warfare. And needed.
Most people "do tongues" for personal reasons, or in some particular ministry situation. The languages they speak can be either languages from around the world, or a personal language understood only by God, sometimes called the language of angels. Some people actually have the gift of walking into a foreign country and speaking the praises of God in that country's tongue. I've often found that quite amazing! For the normal people like us, it's difficult to know what language we're praising God in.
I once heard the story of a man who was frustrated because he only had one "word" in his developing prayer language. He said it over and over. Year in and year out. It's the only word he ever had. But he prayed it faithfully.
One day he was praying with a man from India, along with other men in a van outside an IHOP restaurant in Miami. As the prayers increased the man said his usual word, just like he always did, just under his lips. The Indian's eyes widened with astonishment.
"What did you say?
The man sheepishly said the word again, followed by a meek apology that it was the only word in tongues he ever knew.
The Indian's eyes filled with joy. Out of all the dialects found in India (I've heard there are over 2,000) this man was speaking in his own language and dialect! And not only that his "word" was expressing praise to Almighty God and exalting his attributes in the highest, most distinct ways, with perfect linguistic accuracy.
So we really never know. And we don't really need to know, either. That's where "faith" comes in. And, as we know, without that it is impossible to please God.
O God, we thank you for you gift of the Holy Spirit who nurtures the saints and evangelizes the world. Raise up in us your gifts in us - especially tongues - for your glory and power to be revealed in our lives. Remove from us any blockages in thought, pride, or theology that would restrict us form receiving all you have for us. In the Name of the Father, Son, and Holy Spirit we pray. Amen.