Not heresy at all - Sandy M aka helmar uses a machine of course he's 80 years old
Just teasing Sandy.
Kenny Rowe is another master sheath maker who uses one.
Now as to are there special models for sewing leather - yes.
Actually there are two distinct types of leather machines - sewing machines and stitchers. The difference is that a sewing machine is essentially a very heavy duty industrial type machine and a stitcher uses both an awl and a needle. If you wish to sew with linen thread you pretty much have to go with a stitcher. The fly in the ointment for most of is that a good quality machine capable of sewing three plus layers of 8 oz leather costs upwards of $1000.00 and a stitcher is going to run $3000.00 plus (of course one may always find a deal but in general this is the prices you can expect to pay).
For sewing machines I have it on good authority (recommendations by professionals/friends in the business) that the following are good machines :
The Tippman Boss sold by Leather Factory/Tandy
The Ferdinand machines -
www.ferdco.com - I would suggest the Baby Bull as the most applicable machine for sewing sheaths.
The Artisan machines -
http://www.artisansew.com/Default9.htm -their 2618 model is the best bang for the buck.
Adlers and Jukis are also fine machines although I've heard from various sources that the Adlers can bea pain to keep timed.
All of the above machines can often be found used/refurbished.
As for stitchers - one stands a head above all the rest - Campbell/Bosworth. Others available are Landis and the Union Lock Stitch which is made new (it's a production machine though and is a bear to get to slow right at slow speeds).
Here's a link for more info ont machines:
http://www.a1sew.com/saddle.htm
There are also flatbed leather machines but they are not the best suited for sewing sheaths.
Bottom line - IMO unless you plan on sewing LOTS of leather handsewing is a better bet. I can sew and continue to sew lots of leather by hand (and not just sheaths - I do many, many lined belts every year and stitching a knife sheath of holster is a breeze by comparison) and don't have a sewing machine loan hanging over my head (of course since I do repro work a stitchern is a must - poly thread just doesn;t go on 19th century leather).
BTW if you are going to sew holsters as well as sheaths it is good advice to get one with a holster foot. If you are also going to sew belts than a flatbed machine is also in your cards, although you can sew them on a cylinder arm - it's just easier with a flatbed.
Also be aware that it takes a different set of skills to sew on a machine (voice of experience) - it is not just a matter of sitting down and sewing. In fact for sewing sheaths and holsters I find it much easier to sew and get a GOOD seam by hand.
Anyway hope this helps.