I would add that the 1.5 and 8.5 frets come in handy if you play a lot of very old music, such as medieval and renaissance music. I have found that those genres tend to have 'accidentals'... occasional notes that pop in or out of major/minor key creating an unexpected edginess to the tune. I have also used those two frets for unexpected minor/modal notes in older traditional American fiddle tunes which can be wonderfully quirky.
But if you don't think you'll be playing any of the types of music mentioned by Dusty or myself, I'd say you can be pretty safe just getting the 6.5 and 13.5 pair of frets... those two frets will give you lots of future-proof options.

The question of extra frets comes down to what kind of music you want to play. If you are going to play traditional tunes in a drone style, you don't need extra frets.  You will get used to re-tuning your melody string to get the mode needed for each song.

If you are going to play with others, especially those who play chords, if you are going to join workshops and jams and learn songs from other people's tablature, you will want the 6.5 fret.  The 6.5 fret has been standard now for several decades, and most tablature assumes you have that fret.  The 13.5 fret is just the octave version of the 6.5. I would suggest getting it so that you have the same fret pattern in both octaves.  However, if you are getting one of Ron's small dulcimettes, the frets may be too small up high for a usable 13.5 fret.  Follow his advice on that.

If you want to play blues, jazz, ragtime, pop, rock, and other modern music, you may want a 1.5 fret (and an 8.5 as well).  That is the configuration on my main dulcimers now.  However, remember that we celebrate how easy the dulcimer is to play, and the more frets we add, the more complicated it gets.

And remember that you can always have frets added later on.  My advice would be to get the 6.5 and 13.5 but hold off on the 1.5 and 8.5 until you know for sure what you are doing and why you would want those frets.

1.5, 6.5, 8.5, 13.5.  All or nothing.

You will occasionally encounter DAA TABs which have 6 1/2 fret notes. Don't avoid an instrument just because it has one.

The most important impact of temperaments at the user level is they usually don't play well together. 

 A 6.5 is pretty much essential for Dad and other 1-5-8 tunings, but not needed for DAA tuning, and other 1-5-5 tunings.
In Dad tuning, the 6.5 fret corresponds to the note C#, which is an important part of the D major scale. If you're going to get a 6.5, I'd say a 13.5 is also good.
A 1.5 can also be nice, but anything more than that, I would recommend holding off until you actually have a reason to want them.

The question about temperament is pretty complicated, so this answer will be over simplified. The majority of dulcimers with a 6.5 fret are equal temperament, unless they specifically say otherwise. The goal of ET is to make compromises so that none of the intervals between notes are particularly dissonant in any key. It's basically just taking all the intervals and averaging them out to get tones that are generally inoffensive in any key. When a builder chooses a temperament other than ET, the goal is to give certain specific intervals extra harmony at the sacrifice of other intervals. Usually, this means placing the frets in such a way that is optimized for one specific key. This can lead to dissonance when chording and especially when using a capo. However, temperaments like just intonation and meantone sound GREAT in melody drone style.

Oh,I'd go for a couple extra frets since you're already covered with the more traditional tunings.I have a chromatic that sat in the closet for years while I experimented and learned on the trad instruments.Just in the last year I dusted it off and started figuring it out.Glad I held on to it....nevertheless I'm glad I didn't start with a chromatic,I get such enjoyment out of different tunings,capo positions and string bending.

Hi folks, I'm a relative beginner to the dulcimer. I am drawn to noter/drone playing and have a lovely Dan Cox dulcimore I generally keep in DAA and play solo. I also have a recent Warren May in poplar with (I think) a 6.5 fret but no other additional; I use this when playing with my group. I usually keep this in DAD and without a noter (although occasionally will use one). 

First question: I don't really know what the "temperament" of the May is, and if that makes a difference if I tune it to DAA or other configurations. I don't really understand temperament I guess.

Second question: Ron Ewing is building me a dulcimer right now, and I'd like it to be as versatile as possible- good with various tunings, good for noter-drone, finger style and maybe simple chording. Besides a 6.5 fret, should I get a 13.5? A 1.5? I'm somewhat a traditionalist but would like to not limit my options too much early on. I  am certain however that I'm never going to want to be playing tons of chords (having failed at guitar, banjo and ukulele!) but also it's prob easier to add frets in the future than take them away.

could be there's no "Goldilocks" but just wanting input from folks who know more than me! Ron, if you're reading this post, please be patient with my questions 😉