For rare, expensive and commonly forged material, inscriptions help prove authenticity in my view. I would not purchase a Ronald Reagan unless it was inscribed, for instance. He's just too easy to fake otherwise.
For common material, I do not want personalization. If you pay $30 for a signature at a show and it's personalized, it has zero resale value. If you want a personalization for sentimental reasons, fine... but just understand if you ever want to sell it, a personalized "show" autograph will have no value.
If I'm not mistaken, inscriptions can mean more than simply personalizations. In that case, they could actually enhance the value of an autograph significantly.
As mentioned, I had Rodgers add "SB XLV MP and Heavyweight Champ" to his photo. I have an Edward Jame Olmos Blade Runner photo where he added his character's last line from the film. I had James Tolkan (BTTF's Mr Strickland) sign a photo with "SLACKER" written across the front in big bold letters. I would think that certain more unique inscriptions could make items more desirable.
Oh absolutely... there is no debating that unpersonalized inscriptions add tremendous value and desirability. Always get more when you can.
However, I thought the intent of the original poster was to ask more in terms of personalizations.
Here is a personalized and incribed item from my collection. The "Apollo 11" inscription is quite uncommon and adds tremendous value, even though it is personalized. I suspect I'd have no trouble selling this if I wanted to. ;-)
I have a couple examples of inscriptions that have gone amusingly wrong. U2's Bono apparently tried to write "Glad you're back" on my CD, though it looks like "Glue your Brussels."
Packers safety Nick Collins botched an inscription request big time. I asked him to write "Picked off Big Ben . . . dove in for the TD . . . Best Moment Ever!" What I got was "Pick off Big Ben . . . Drove it TD . . . Best Moment Ever!" My wife started laughing when she saw it.