It's not always possible to predict in advance whether a pitch may develop into one which takes spin later on. And if it does then a fingerspinner may very well be a better bet than a 4th seamer, and a much better bet than a 5th seamer.
While it's obviously true that you can never be totally sure, in my experience a pitch that turns little to none on days one, two and three then turns significantly on day four and five (or something along those lines) is extremely rare nowadays. The rate of pitch deterioration is sufficiently slow (and this evidently didn't used to be the case) that over five days most pitches will either turn a good deal from the start or won't turn a good deal at any stage. I can think of almost no cases in my cricket-watching time where a fingerspinner has achieved little purchase at the start of a game and gone on to achieve significant purchase later in it. Not to say there aren't any at all, just that I don't really think it's wise to make a selection to cover that possibility when that possibility seems to me to be an extremely remote one.
I don't think that you can rule out the variation point either. I've no idea how one would prove the point by use of statistics, but I can't help but feel that the variation offered by a spinner can be useful in changing the flow of a game - either by taking a wicket or stemming the flow of runs.
As I say - I've seen precious little evidence all my cricket-watching time to suggest this either. If batsmen are going to make a mistake, they seem to me to be as likely to make it against an innocuous seamer as an innocuous spinner. There's absolutely no way to prove this though - either way.
And also there's the case of...
The issue is not whether a fingerspinner has more variation than a seamer, the question is whether a bowling attack containing a fingerspinner has more variation than one without a fingerspinner - whcih self-evidently it does.
Yes, it does. But the point is that a seamer has more attacking tools at his disposal - to be able to increase his likelihood of getting a batsman out rather than a batsman getting out purely and exclusively through his own error - than a fingerspinner does, on most surfaces. It's also self-evident that bowling at even 70mph, never mind 80mph, gives a batsman far, far less reaction-time than bowling at 50mph - and that's just the start. Seamers can do so much more with the ball, on most surfaces, than fingerspinners can. And given the pace they're bowling at, they need to do so much less in order to make a delivery difficult to play than a fingerspinner does. All of this just adds-up, to me, to making your stock-in-trade, reasonable-quality seamer quite a bit better than your stock-in-trade fingerspinner, or even an exceptional fingerspinner, on a surface which does not allow the fingerspinner to turn the ball greatly.
All of which said, I do have a lot of sympathy for your basic point.
Why, I thank you kindly.