This is a great question that comes up often.  I think it depends on the market, the neighborhood, the layout of the house.  

If your target market is families with young kids and there is another bathroom with a tub for the kids, you are probably okay because tubs seem to have fallen out of favor with younger folks.  

Older folks, though, seem to still want a soaking tub and separate shower.  So, my answer is like everything else, yes, it will reduce your buyer pool by the number of people for whom a tub is a must have.  If the bathroom is large enough to redesign and put one in, that will work for some people (but not those who won't consider doing that).  

If you have ONLY a shower, then my suggestion is it needs to be fabulous. ;-)

My Female buyers always point out the lack of a bath tub in the master bathroom. But if it is the perfect house other than that it really has not mattered in the end. 

Great Question 

Buyers I work with here, especially the seniors, prefer a shower and do not care as much about the tub in the master as long as the other bathroom has one.  Still, why not design with both and never let it be a barrier when selling?

Ironically, after reading this question, I saw a survey on remodeling trends for 2015 and it indicated tubs are making a comeback...I could not live without one.

No, I've actually had buyers say it is a feature.

Debbie, I'm seeing it matter less and less as long as the shower is nice (not one of the plastic inserts, but tile, etc.).  As long as there's a bathtub in the other bathroom(s) for bathing the young ones it seems to not be an issue.

I want a bathtub, but most of my buyers don't seem to mind. I would certainly include one in a nice master bathroom...especially if it's a higher end home. A jetted tub is also a bonus. Buyers seem to take note of those and add that feature to their lists of positives. 

Debbie,

It really depends on the marketplace and the buyers.  Personally, I like a big shower, and don't care about the tub all that much.  In Napa our shower overlooked the hills, and we also had a large jaccuzi tub, which Ron like also overlooking the hills...If push came to shove, I think he would forego the tub for an outdoor jacuzzi, which we both would us.  I think in the high end range, people want it all, even if they don't use it, like the to die for kitchens which still have the original tags in the oven. LOL A

In nicer homes both a shower and a tub are still expected even though the tub is rarely used.

Deluxe showers are in demand these days.

If the shower is excellent I don't think it's a hindrance

Goes without saying, Debbie...

Debbie, I'm guessing there is probably at least one person is each couple who would find a shower-only to be a deal breaker.  I think we have to think about our someday buyers when we renovate - something that I didn't do when I redid my own house.

It would for my tastes. I think a tub in a MBR is essential. 

If the home is big enough to accommodate a tub in the master bath room, in my opinion it should definitely have one.  Despite the fact that many people rarely use a garden tub or jacuzzi tub, I think they add resale value to the home.

It depends on the price range of the house. In some prices as long as there is another bathroom with a tub people do not mind.