Taking a whole spec for a single skill is rarely worth it mathematically, only do so if you actually want to use the talents in it. There's other ways to get Lightsaber as a skill too: Some trees get a talent that grants Lightsaber and Lore as skills, and there's the Well-Rounded talent which grants you any two skills as career skills. If you just want to get Lightsaber to rank 2 or 3 and not actually do much with a saber beyond that, either grab the skill with one of the talents if they're in trees you're interested in, or take the EXP penalty. 10 or 15 EXP setback from buying 2 or 3 ranks won't break the bank, usually.