There are pros and cons.

Pros - you can hear your baby's heartbeat whenever you want, which gives you peace of mind. This is a huge pro, I know, especially when you've had a previous loss. It might help you avoid unnecessary trips to the maternity ward or your doctor for reassurance. Your family can join in the pregnancy more actively by hearing the baby. Towards the end of the pregnancy you may be able to judge when the baby is engaged, because the heart is positioned lower.

Cons - the results of overexposure to the doppler's frequencies has not been established, and outside of clinical necessity, I'd be leery of that.

You aren't qualified to interpret the results - on hearing a heartbeat you may assume all is well, but there are variations that need to be considered, and particularly if there is a lack of foetal movement, there may still be good reason to consult your physician.

You may not learn to relax and enjoy the pregnancy - there is a risk that you can only be satisfied everything is okay when an external test can prove it to you. I believe this is a risk for women who are being subtly pushed away from an attitude of seeing birth as a natural and healthy process.

You may end up with even more stress if your baby's heartbeat is difficult to find due to an anterior placenta.

I think there is a lot to be said for learning thinking and relaxation techniques to help deal with the inevitable stress and anxiety of pregnancy, and I think this has overall benefits, and I do see the doppler as a bit of a bandaid measure rather than dealing with the underlying causes of the anxiety. That said, having suffered pregnancy loss myself I can well understand the temptation of having something like that on hand.