This question is very similar to another one, although the latter deals with new lenses on old bodies.

First of don't listen to your friends telling you that Canon or Nikon is the way to go. All of the DSLR/MIRC manufacturers today make cameras that are capable of producing high quality photos and the limitation is the photographer unless you're extremely good. A person that is so skillful that he/she is limited by a certain camera brand knows this already and doesn't have to ask this question.

There is some truth in the argument regarding older lenses not being able to keep up with newer high resolution digital bodies. Older lenses were designed for use with film in an era where people weren't pixel peeping so older lenses are often not as sharp and have increased amounts of chromatic aberration when compared to lenses designed for digital cameras. Also digital sensors are more reflective than film, hence the coatings on the rear element of modern lenses to prevent these reflections lowering the contrast of the image, something that older lenses lack.

These aspects are not the only ones to consider though. Even new lenses might have trouble keeping up with the high resolution of the Sony Alpha SLT-A77 unless they are really sharp. Also, properties such as maximum aperture, quality of the bokeh and build quality may not have changed much. If you already own some expensive lenses, albeit old ones, you have to consider the cost of getting the same lenses for another mount. The lenses might be able to be used with adapters on another system but at a possibly lowered functionality.

The older lenses are likely not keeping up with the extreme resolution of the Sony Alpha SLT-A77. There are other aspects to consider though and even newer lenses might have trouble keeping up with this resolution.