How do or could radio telescopes contribute to exoplanet studies? Current and upcoming telescopes which find exoplanets and take spectra of their atmospheres seem to be optical or infrared. What about radio telescopes applied to exoplanets? They are larger and more easy to use with interferometry. Why aren't radio frequencies as useful as optical/IR for exoplanet studies?
Radio telescopes are frequently used to observe the births of stars and their planetary systems. The longer wavelengths are able to penetrate from beneath the envelope of gas and dust that shrouds any attempts to view these events at optical or infrared wavelengths. Unfortunately, the smallest angular resolution of a telescope goes as $\lambda/D$, where $D$ is the telescope diameter and $\lambda$ the observation wavelength. As radio waves are $10^5$ to $10^{6}$ times longer than optical waves, then one needs enormous telescopes to get similar resolving power. This is attempted/accomplished by constructing arrays that can act as a single aperture. 

Whilst interferometric techniques to gain spatial resolution are certainly possible, if one wants to directly image exoplanets then optical/IR is still the way to go because this is where most of the intrinsic light from an exoplanet is found.
It is possible that planets orbiting stars may emit radio waves in their own right, through charged particles accelerated in their magnetospheres. Jupiter, for instance, is a strong source of radio waves in our solar system, but would be very difficult to spot at the distances of exoplanets. Some searches have been attempted, but nothing yet found (e.g. George &amp; Stevens 2007).

Of course transit observations do not require superb angular resolution, but they do require lots of signal and precision. Levels of precision that are routinely achieved by optical telescopes (especially in space - e.g. Kepler) are not possible with radio telescopes, and the parent stars themselves are weak, variable radio sources, which makes studying any transit signature very difficult. In fact the main use of transits has been the other way around - to search for intrinsic planetary emission by looking for a drop in flux when the planet is eclipsed by the star (e.g. Lecavelier Des Etangs et al. 2011).  Radio emission might be enhanced in hot Jupiters if the exoplanetary magnetic field interacts with the stellar magnetic field.
This paper does claim to have detected a weak exoplanetary signal from a hot Jupiter, identified by its disappearance when eclipsed by the parent star. However, the result was not repeatable.

The SETI programme targeted a set of Kepler stars with evidence for a planet (or planet candidate) in the "habitable zone". The initial searches for narrow-band radio emission (at 1.1-1.9 Hz) appears to have concentrated on those systems containing multiple planets.

The (null) results for $\sim 100$ system were published by Siemion et al. (2013). The observations were not sensitive enough to detect the sort of "radio chatter" emitted from our planet, but might have detected some sort of deliberate signalling beacon.

A slightly more imaginative approach is to observe Kepler multiple planet systems when two planets are in conjunction as seen from the Earth. There is then the chance of seeing communications from one planet to the other. (e.g. http://adsabs.harvard.edu/abs/2014ebi..conf..5.2K ).