This might be the best Seeker card in the entire game. Not only does it give you +2 to your investigation skill test, it will more often than not award you with 3 clues. 3 clues for a single action is a huge time saver and well worth the use of one card and 2 Xp. You can pretty much vacuum a location for all traces of tentacles prints and black candles and still be able to move away the same round.
Actions are pretty much the life blood of the heroes. Saving time is what will win the day, and this card is a huge timer saver. Even more so with Rex Murphy, since Deduction mimics his amazing ability to find extra clues.
But, is it the first card you should upgrade in a campaign? Probably not - the basic versjon Deduction (+1 Knowledge) will net you 2 clues; enough for most cases. If you play a Seeker hero there's bound to be other weak spots in your deck you need to cover first - like being able to kill at least a few monsters on your own.
The card is also less likely to net +2 Clues in a Hard or Expert campaign, but the +2 to the skill test is also more likely to be vital to your success. I think it's worth it.