For players who stick to a main character and never spend too long on alternate characters, Subclassing is huge.
Finally, players will have a reason to buy DLC Classes if they originally made their main character one of the base game Classes. Those DLC Classes have skill lines which can now be learned by their main character. I'm looking forward to getting an 8th Class because I'll be able to use its skill lines on my main character.
Further, this means Skill Styles for Class abilities will be valuable. For instance, Summon Winged Twilight, Warrior is now useful for non-Sorcerers if players decide to pick up the Daedric Summoning skill line. Maybe we'll get Skill Styles for other Class abilities that'll entice us.
Making Subclassing a base game feature is a very smart move, in my opinion. This will encourage people to spend more time with the game and spend more money on new Classes and Class Skill Styles in the future.
PC/NA — Lone Werewolf, the EP Templar Werewolf
Werewolf Should be Allowed to Sneak
Please give us Werewolf
Skill Styles (for customizing our fur color),
Grimoires/Scribing skills (to fill in the holes in our builds), and
Companions (to transform with).