Either you merchantile like mad or there's no way you're leveling it without training. This is a balance issue as all skills should level at an equal pace. The leveling system (level it by using it) and the amount of speech checks in the game is an absolute recipe for disaster. While combat skills and crafting skills get used nonstop, speech checks actually feel like a rarity. The amount of speech checks in game is totally disproportional to the leveling system itself.
2) It's usefulness is trivial
I get approached by the same "radiant" thief encounter I've seen a dozen times, and I can either make Speech useless by simply being a thief myself OR....I could fight him. He's not that hard.
And every boss fight in the game? Are there ANY of them that encompass speech? I mean, some random [censored] with a dagger approaches me and I can talk him down, but the big level 80 boss character with the OHKO spell can't be talked down at all. Some might say "realism da boss isn't scurred" but no, the idea is YOU are so good at speech that you can persuade others to listen to you, and from a balance perspective it SHOULD offer you a chance to pass up on tough fights. New Vegas did a wonderful job making speech useful in that it -literally- unlocked quests (couldn't take all of them without speech) and got you out of some incredibly difficult fights. (Ulysses, Lanius, the BoS firing squad that literally has five snipers pointed at you the moment the fight starts if you fail the speech check)
"The merchantile effects are useful though!!" Yeah, they would be if Draugr and Bandits didn't literally s**t gold.
3) Even the dialog is lulzy
Nothing major I suppose, but for being persuasive, it's certainly pretty lame. I remember one quest in particular where I had to convince a drunken guy who owed a woman debt that he didn't have to pay it back because she pitied him so much (weird concept to begin with wtf?), I approach him with the issue and his reaction is "NEVER! A -INSERT FAMILY NAME HERE- always pays back his debts!!" My reaction? My ingenius smooth-talking response in the speech check? "C'mon, don't be stubborn." C'MOOOOOOOOOOOOOOOOOON is what I basically said to him. His world as he knows it then shatters from the foundation as he has a life-changing revelation and realizes the error of his ways, apologizes and vows to change his ways. All because I said "C'MOOOOOOOOOOOOOOOOON!"
Lolrly? Pretty sure normal conversations don't work like that.
I'm in no way advocating such a cut when I say this, but at the state speech is currently in? They might as well cut it. Which as I said, I don't advocate this. They shouldn't cut it because to be blunt, it's downright shameful that an RPG can't properly encompass a speech skill. Passive characters have existed in RPGs for ages.