This perception may have been true a few years ago but is certainly less likely today. What's more likely is that a developer sees a real or perceived obstacle in implementing or developing a widget with one library over another. "Either the widget exists but I can't seem to install it or it's missing and I'm having trouble building it."
Either way, I'm beginning to develop more of a purist's view -- choose one framework for a project or environment and stick with it.
I've developed a few sites that embed the Gallery Photo Manager in Drupal. At its core Gallery uses YUI while Drupal uses jQuery. To complicate matters I implemented a Script.aculo.us-based Lightbox effect. The use of Script.aculo.us introduced repetition in basic DOM handling functions that are already provided in YUI and jQuery. Although the repetition only adds up to a few extra kilobytes, these kilobytes add up.