Im not sure about that. I mean yes if there aren't many $5's you need lots of, then Saboteur can be a nice pickup.
Or if you just need an attack, and it's the strongest one available.
It won't lengthen the game for most of the times. Yes, hitting a good card can slow your opponent down, but it also doesn't provide anything. In RL, I used to hate it, because i mostly play 4-Player-games, where triple Saboteur just kills your deck, more or less regardless of what you're doing. Most of the time, it comes down to destroy the other Saboteurs and the one or two players, whose Saboteurs survive longest, will win the game. This is just so painful and no fun at all, even if you win (because it depends so much on luck).
In 2-player-games, Saboteur is not nearly as strong. Especially early on, it's mostly crap. You'd rather pick one (or two) up as a payload to your engine/cycling deck.
In 98% of all openings, I'd rather open Militia than Saboteur.
Btw, opening Rogue doesn't appear to be useful, either. But as gainer, it's quite nice. I agree with KingZog3 on this.