I'd say the small problem with vendor gear is that it can a bit pricey for the average player. For some, 100K just to get a full gear set for 2-3 dungeon levels is quite a steep price to pay.

And if we go by the auto item drops, it benefits tanks and healer more due to faster queue times for SB leveling dungeons, so dps is usually going to have to wait longer for this route.

The Shire gear and its augmented versions from the poetic tomes are basically ways to get started and can be used as a compromise for gearing up if gil for vendor gear and/or time for SB leveling dungeons are not suitable for the players.