Not daft at all !
Don't forget that alternates may be located beyond the original destination anyway which would preclude somewhat the reason for the diversion. In the normal course of events planned 'contingency' fuel should cover unexpected headwinds, lower than planned flight-levels etc - however, on the day it is possible that things totally conspire against the crew in question. In such event an analysis of weather at destination, anticipated delays, possibility of short-cuts and an expeditious approach may help the crew out of the mire - an appreciation of weather and similar factors at the planned alternates will also come into the equation.
As the crew will be monitoring their fuel situation (I hope) during the cruise they will also have time to weigh up the reaction of their management to a. landing at destination with less than normal reserves or b. diverting en-route to refuel because they cannot justify continuing to their destination - either way they will be criticised or worse !
Some aircraft and indeed some airlines are more likely to encounter this particular problem than others. Personally I weigh up all the things may might go wrong and come up with what I believe is a sensible fuel figure which strikes a compromise between the plog (computer flight plan) fuel requirement and the cost of carrying fuel that I probably will not require on the day. Fortunately my aircraft allows me the luxury of taking as much fuel as I wish 99% of the time - other aircraft types are however much more performance limited and may operate schedules in which the likelihood of an en-route diversion are much higher. Dare I say that since the introduction of computer flight plans the carriage of unnecessary fuel has reduced dramatically - which is not to say that the figures don't have to be carefully examined against met-winds etc - rubbish in, rubbish out !
There are of course additional factors regarding long-haul operations which are probably outside the remit of your question.