they're not the end-all, but i use them to fill in the last few bench spots of my roster. while spring stats dont necessarily mean greatness, they can give you a clue of someone who might break out this year.
for example: who'd consider putting mike cuddyer on their bench before this spring? nobody. but when he's hitting .400+ 4 HR 16 rbi and slugging .700? shoot i'm inclined to give him a shot.
last year i was all over tony armas jr for the expos, mainly cuz he had 27K and 3 BB all spring. he was awesome early last year, and he only had one bad start out of like 4 or 5... then after that bad start he needed shoulder surgery and he was out for the year.
but i swear to you, armas was pitching like a madman... so in spring i look for pitchers with staggeringly good #s, hitters who are going apeshit, and if they're previously menial players who nobody will pick up, f0ckit, i'll give 'em a chance to shine on my bench and perhaps into my starting lineup... you never know. it's a gamble, i mean you cant realistically expect to fill a starting position with someone previously unproven who had a hot spring, but i think they do give you ideas, moreso with pitchers than anyone else.