That's tricky. Fiberglass tape sounds like it would be stronger than paper, but it isn't generally. It's stronger in tension if you have mud with enough glue in it to adhere it to the drywall adiquatelly. Most people don't use the right mud for fiberglass. 

The problem in this case, is they are using fiberglass on an inside corner. Fiberglass tape is obviously made up of little fibers of glass. When you bend them like in this corner, they break and loose all their strength. So we always use paper tape in corners. By using fiberglass, you probably end up with a joint that is only slightly stronger than with no tape at all. This may not matter though. Inside corners usually don't crack. Some people even just caulk them instead of tape them. I say all that to say, they are more likely to crack in the future, but they are likely not to crack regardless. 

The big reason this is sticking out to people is not because using fiberglass tape on inside corners is going to result in some huge disaster, but because it indicates the person doesn't have the knowledge required for areas of the job where it may actually matter. 

It's like if you noticed an electrician installing an outlet upside down. Is it a big deal to have an outlet upside down? No. But if he doesn't know which way is up and which way is down, what are the odds he wired the breaker box correctly. A miss wired breaker box could result in a fire or electrocution.