I suppose it comes down to a personal choice. While you CAN sharpen tools on your lathe with a grinding wheel, why do that and risk your lathe when other machines and methods can do it better? A bench or belt grinder, a little skill and you're done.
A tool post grinder is an attractive attachment but it is most commonly used to produce precision dimensions on hardened material. How often do you do that? They can also be used to true up chuck jaws; how often do you do that? I don't own a tool post grinder but it was an intellectual choice not to buy one because I weighed the risk to my lathe vs the situations listed above.
If you own a tool post grinder and wish to take the chances with your lathe then go for it. Lots of other hobby guys do this so you aren't alone.