I had the original Gen 1 Time Capsule with the well-known failing power supply problem. The problem with the TC setup compared to AEBS+Ext drive is that if the TC fails in any way (like the power supply), then you lose your data with it. Yes, Apple replaced my TC, but I lost the data that was on it.
With AEBS+Ext Drive, you lose your data if the drive dies. But not if the AEBS dies.
Now I keep backups on an external drive attached to the TC. I use the TC internal drive for temporary storage. Sure Apple probably dealt with the original design problem. But I don't trust my TC anymore.