I am not a coach , just a beginner Tri-athlete , but I started cycle timetrialling in 1978 so I have ridden more than a few and understand a bit about pacing. I don't have a power meter , I came close to buying one a few years back but never took the plunge, put off largely by the cost. I think a power meter is a great training  tool if you are prepared to take the time to understand how to use it properly and understand what power levels to ride at. To that end a coach who understands power meter use is invaluable and conversely I know a lot of cycle coaches insist or at least prefer their clients to have a power meter as it aids training prescription and avoids a lot of the issues associated with training with an HRM. A cycle coach can test you and help advise on appropriate training levels, possibly even give guidance on a range of power to use when racing. ( at a minimum get the "Book Training and Racing with a Power Meter" ).

For racing though knowing precisily what power level to race at to get the best result on the day is really very hard. It is hard enough to determine for a pure cycle time trial so I imagine trying to determine what power level to use in a triathlon is even tougher depending I guess on how much effort you expended in the swim and how much you want to save for the run. At least with a pure time trial you don't have that issue but even so I know of a lot of top riders who will not use a power meter for racing. Sure they have a power meter and swear by it for training but as a racing aid it is not ideal. Much better to ride on feel , sure havea power meter and HRM on the bike , but cover the display up and only use the data for post event analysis. If you try and use it in the event you stand a good chance of either over or under estimating the power you want to ride at and at a minimum it will be a distraction from concentrating on your breathing and how you feel.