symphx
Fractal Fanatic
Off the top of my head...
Service. As others have mentioned prominent artists on tour can have repairs expedited and loaners provided, not to mention having staff from the company personally helping with setup. Having Matt come out and help you (or your tech) dial in sounds has got to be worth something.
Exposure. Sure, Steve Vai has helped to sell a few Ibanez guitars, but I'll wager Ibanez has helped to sell a few Steve Vai albums too. If Fractal want to run a full page ad in Guitar Player magazine with a picture of me and a caption saying "I use the Axe and so should you"... I would be pretty OK with that.
Good will. If a guy like John Petrucci tries the Axe out and thinks it is pretty good, it costs nothing for him to say a few words to that effect and allow Fractal to put it up on the website (other endorsments notwithstanding). Guaranteed many of these guys are kinda gear nuts, and when helping the smaller manufacturers succeed costs you nothing it is easy to justify lavishing a bit of praise publicly on something you think is cool. Heck, the rest of us are quick enough to share our enthusiasm for our cool toys when given the chance, why not the "big boys" too.
All this before even considering discounts, swag and freebies.
On the other hand some of these Artists could use the help, and are bigger in fame then money, guarantee Jeff loomis makes less money than Cliff Chase, but is better known! So for nothing he's being pretty nice...(course the real small artists are gettin free pub)