I believe you will still be able to utilize T-Mobile's insurance. Its not like its an At&t or Sprint specific phone then it would be understandable why T-Mobile won't cover it. T-Mobile sells the very same phone as the one on the Play Store so it shouldn't be an issue.
Note: If you do decide to buy the Nexus 5 from the Play Store then you'll be covered under a manufacturers warranty. Its not exactly the same as T-Mobile's insurance but it will cover any software and/or hardware defects (minus a cracked screen or water damage).
sent from my neXus 5