I think this depends on one's chosen main instrument (if there is one) and one's aspirations. For example, singers can not practise for that much time in a day (the human voice gets tired) so to become an excellent singer one doesn't have to (or can't) practise for all of one's free time in a day (I believe the same is true to a lesser extent for some brass instruments); however if one is at the Julliard studying piano one is advised to do no more than five
hours a day practising (apparently any more risks injury). So if one wants to be a world-class singer then other instruments will probably help as it means one can get more experience and practice of music, if not singing, into the day than one could otherwise without the other instruments; however, if one was taking, say piano, and wanted to become a world-class performer playing another instrument (seriously at least) would take away from one's practise time (unless of course one would only practise the other instrument when one would never want to play the piano anyway).
Of course, not everyone is (most people aren't...) willing to spend all
of their free time on their instrument, or has aspirations of being a professional soloist, so if they enjoy to have two or more instruments and that will mean they do the same practise as they could on one; or even to be not quite as good on their instruments so that they can play more instruments that's a reasonable choice
. I do think though that once the number of instruments gets really large, one has to guard against becoming a 'jack of all trades and master of none'; having said that if one wants to become a composer that's probably quite a strong position to be in.
Personally I'd like to play fewer instruments and be able to dedicate more time to the ones that I want to play to get better at them; but that's a matter of personal choice and some people will feel that they would rather have a wide interest in a number of different instruments than focus themselves on one or two either purely for enjoyment, or for pragmatic reasons: it's all about personal choice