On the one hand due to the relative small size of the site; it is perfectly feasible to roughly keep track of all questions that get asked so increasing visibility seems not that important; or if this is needed the current system works well enough and an expiry date is desirable.
On the other hand due to the nature of the subject. IMO the only type of question where a bounty is really useful is 'boring but somewhat tedious.' If I find something interesting and can answer it I will do so. If I can answer something easily I will do so. The question is what do I do if I could answer but find it tedious (where tedious is everything above several minutes); then those points might or might not be a motivation (actually that I'd gain the points would not be that much of a motvation but that somebody is willing to spend them might convince me of the relative relevance of the question and motivate me; yet then it is also sufficient I once saw them and then anything I can do but do not find the time to do within seven days is mostlikely so tedious I would not do it anyway).
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