Should a tester acquire domain knowledge from outside to gain an advantage over others and give that supposed golden tinge to his resume?
If I have to answer this in one word I would say No.
The priority in my view goes to testing and technical (programming/scripting/tools) skills. So the tester should focus on what/how/how much to test so as to bring out information and unwanted defects faster and better than ever.
To become a better tester – he needs to listen to and learn from testing masters (one must listen to his inner self which becomes better with time if one consciously builds it and feeds it – one must never be a blind follower – Beware! the testing planet has groups bordering on cultism) be a part of testing forums, read and write about testing experiences, be open minded to changes and talk every night with oneself noting the rights & wrongs….. While he learns testing and gets better it would help if he understands the world that the developer lives in and picks up understanding on good coding practices, common development mistakes and gets acquainted with their thinking, what makes him proud or queasy and so on… The tester would benefit immensely by learning some scripting language and gets the developer to mentor him and review his scripts. This bond would help not only him professionally but the team as a whole because the scripts can then be put to interesting uses like populating data, writing, reading logs and pick out errors and so on.. While getting on back-slapping terms with the developer the tester has to keep his pessimism and the reason of his existence uppermost in his mind and not fall into bad habits that the ‘smart’ developer might influence him into.
As far as the domain skills are concerned – when the tester gets into testing for a project – he must learn everything about the domain allotting time (through the supervisor of course) to it such that he does very good justice to it while performing the other testing activities. When learning the domain he must read the documents available and speak with the business users. He must make copious notes, share, brainstorm with colleagues, ask lots of questions, (learn mind-mapping) and so on… Good knowledge of the domain will ensure he tests by delving more deeply into areas that the business users would be looking critically at.
With experience the tester would have domain knowledge not really by choice but say because he worked with organisation(s) which catered to that segment/vertical or he was allotted a project on purpose with a domain that he has worked with previously. The collected expertise of the tester in domain and his big-picture knowledge opens the Business Analyst role also for Testers which is a new avenue to trudge… I think this is better than spending time on learning a domain by time-slotting it as against spending priority time on sharpening testing / technical skills…