I’m a member of the Assemblies of God, and yes, as a Pentecostal denomination there are a number (perhaps even a majority of adult members) of the A/G who do speak in tongues at one point or another.
The thing that’s interesting about it is that there isn’t a lot of teaching on the topic, at least not Sunday morning pulpit types of teachings. But the general consensus, as far as I’ve ever seen (I’ve been a regular congregant at no fewer than two dozen different Assembly churches over the years, thanks to moves.) is that there are two different flavors of speaking in tongues.
The first is the combination of tongues and interpretations as a tool of prophecy. This is not, I’d hazard, what the OP heard the TV evangelist speaking about. It’s a fairly rare occurence and everyone is well aware of the risk of a.) kooks who rattle off something that isn’t tongues and/or isn’t meant to be interpreted, which leaves the rest of the congregation stymied and embarrassed and leaves the kook angry that no one was “receptive to the spirit” enough to help bring forth their word from the Lord and b.) false “interpretations” where someone gives a statement that has nothing to do with the real prophetic message that was to be disseminated. (Yes, it’s a big leap to say that there is a prophetic message involved in this kind of incident, but it’s a flat out matter of faith.)
The other type of tongues, and the one that the OP had to be hearing about, is something that has been referred to as a personal “prayer language,” an evidence of the indwelling of the Holy Spirit, by which an individual’s spirit can directly communicate with God without language (vocabulary) barriers and self-editing. Use of personal tongues in this manner can happen during any prayer (personal or group) or during any time of worship, and is referred to as “praying in the spirit” or “singing in the spirit.” (The latter phenomenon can be quite amazing when it happens in congregational services, as everyone begins to sing a song with as many different words and different tunes as there are people singing, but in perfect blend of harmony and rhythm.) For those who believe that they have received this particular manifestation of the Holy Spirit, it’s something that they can turn on and off at will, much as they might switch to speaking another known, earthly language, they can simply begin to pray or address God using their prayer language.
In the pentecostal tradition, prayer is a means of communication which is not merely one-way. As such, it is a crucial part of one’s spiritual development and should be undertaken at every available opportunity. Given that, it would of course follow that if praying in tongues provides a more open or honest means of revealing ourselves to God in ways that we could not otherwise approach, then the evangelist was correct that praying in tongues provides spiritual benefits which are important enough – to those who believe in this mold – to seek daily.
I have to say that this is not something I ever expected to get into on the SDMB. Whew. I think I need to go pray.