An honest man doesn't argue the economics... although another pen fancier has actually worked out that over time, a fountain pen works out as a better deal (which he lays out in point #3, here).  When looked after, fountain pens are amazingly durable mechanisms-- the one of  the oldest that has been in my collection was made at some point between 1924 and 1929, and it makes some of the prettiest marks of all my pens.  Taking an environmental standpoint, there is a lot less waste involved in choosing a fountain pen as a writing instrument, as ink generally comes in recyclable glass bottles and it takes a long time to write your way through a bottle of ink and you keep the whole pen throughout its lifetime... or, frankly, your lifetime-- I have to imagine this pen has quite outlived it's original owner.  Compare to this the amount of carelessly discarded plastic in the cheap ballpoint, and how many you might fling in a lifetime of writing stuff down.

That aside, the act of writing with a fountain pen is more relaxing, and more given to making legible writing.  Both the mechanism of delivery and the ink itself in a ballpoint leads to uncontrollable variation of the line the pen puts down-- blobs accumulate on one side of the ball, then drop onto the paper at direction changes, or areas of the ball run dry, making little pale areas; in this sample, the problem is especially evident at the bottoms of the 'y's.  Moreover, a certain amount of pressure is required to keep the ball turning-- someone who has a lot of writing to do can get tired writing parts.  The ink in fountain pens is, broadly speaking, anxious to get out and the mechanism of the feed is there mainly to limit the speed of its departure.  It should take almost no pressure beyond the pen's own weight to start making marks on paper, and the visual density of these marks is extremely regular, as in this sample.  I want to point out that the ballpoint above was a reasonably good one, costing about seven dollars, while the other was a Parker Reflex, one of the cheapest and less well-regarded of modern fountain pens.  One can, if inclined, seek out a pen with a flexible or specially-shaped point to introduce variation, or "shading" into one's writing, but it's then a choice rather than something inflicted by the pen.

Apart from the legibility aspect, I have found that writing with a fountain pen brings a certain notoriety to one's writing.  Some of this is just the reader declaring, "Hey!  I can read this!" but there is also more control over the appearance of the writing.  A less tired hand is a more steady one, flourishes look more natural, and there's a whole spectrum of ink possibilities to suit mood and personality.

A last item, but just a quibble, really.  If you go out and spend a pile of money on a really nice pen, it should stay a really nice pen.  Were I to go out and buy, say, a Mont Blanc platinum-plated Bohème in its ballpoint version:


For all I know it could be a transcendent writing experience.  But, at length, it will run out of ink, and if I'm still broke from buying it (which is likely) I'll nip out and get a no-name cartridge for it.  It will still be pretty, but it won't work the same way.  On the other hand, I similarly buy the fountain pen version of it,

 ...and presently it runs out of ink.  I can refill it with just about any ink; Mont Blanc's own, some history-steeped juice from the flower-mangles of J. Herbin in Paris, or some bog-standard Parker's Quink, and with a very little variation based on ink viscosity, it will write exactly the same way as ever it did.  After all, if you've got thousands of dollars to spend on a pen (and some do), or you want to make a particular gift of a pen, its nature shouldn't be radically altered by the fact it's being  used for its intended purpose.

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