Should I worry that my 11-month-old is only babbling?

No. Babbling is a normal part of expressive language development. Understandable words occur over a wide range of ages as babies develop differently. This may happen as early as 8-9 months of age or normally as alte as 18-20 months of age.
No. No - average age for speaking a word or three is about 12 months of age. But remember that for every developmental milestone, there is a range of normal. For speaking first words that can be anywhere from 9 to 15 months+. Babbling at 11 months is quite perfect with first real words coming soon. Baby should also be demonstrating good comprehension of speech now and progressing soon.
No. Most babies at 11 months are not saying many identifiable words. The receptive language should precede the expressive speech. For example, your baby should be able to respond to his/her own name, but probably will not say your name quite yet. Reading with your baby at this age, however, is extremely important in his/her language development and is the best thing you can do to support that process.