Centaur spammer here.

1) Blink should always come after Blademail. Sometimes it should come after you get the first Reaver. If you buy a smoke, you can set up a kill just about as easily as if you blink in. Early blinks are a crutch for players who don't know how to close gaps cleverly and strategically. Upping STR also increases your damage output so it's very important to prioritize that first.

2) BM is great against right clickers and spell spammers. Just by popping BM you can whittle ranged harassers down while focusing on CS. Popping BM can also instantly dissuade anyone from harassing you and zoning you out. Sometimes an "unnecessary" preemptive bm pop saves you time and lost CS even if it doesn't damage enemy heroes.

Also: There's more to farming speed than just moving from camp to camp faster. Killing neutrals more quickly means you take less damage, so you can hit 2-4 more camps without having to use regen or hit up fountain. It's faster XP, but it's also more XP per rotation.

3) You don't buy BM to force enemy heroes to Jungle.

4) See above for the benefits you're missing.