From a DM's point of view, I agree with you, but I think when you're talking about an animal companion for a PC, it's a bit different. Running ambushes is a pretty heavy playstyle investment for the whole party, and while that can work for some groups and campaigns, a lot of the time, you just don't have that luxury—villains act, heroes react, etc. A hit-and-run style doesn't really work here either; the spider doesn't have any way to gracefully exit melee or ranged combat options, so if ambush tactics are off the table, frontlining's really the only option.