As a Blood Angels player, I've come to know and love our cheap MM attack bikes.
Generally, unless you're facing a lot of Sv4+ or worse armies who don't pack tanks, the Multi-Melta is the way to go.
While a pack of 3 HB attack bikes (each packing a TL bolter that can fire like stationary on top of everything else) can shred weak-save infantry, they're pretty useless against anything with a 3+ save, whereas a multi-melta is always lethal, when it hits. Chances are, against MEQ (Marine Equivalent) armies you're going to score more kills against troops using three multi-meltas than you will with 3 HBs (Quick Mathhammer: 9 HB shots at BS4, 6 will hit, against T4 3 will wound, against 3+ saves, 1 will penetrate. Meanwhile 3 MMs, 2 will hit, 1.66666(call it 2) will wound, both will penetrate. Mathematically, it's just over 60% more effective to use a meltagun against MEQs than a heavy bolter)
So against the most popular armies (Marines, Chaos, 'Crons) the multimelta is not only better anti-troop, but it is, of course, wicked anti-tank. Three MMS, mounted on fast, maneuverable bikes can wreck just about anything. I made my weapons interchangeable, but unless I know I'm facing mid-low save armies, I rarely bother switching out the meltas
BA also tend to lack traditional long-ranged support, so veteran assault squads and honour guards with meltaguns, and MM attack bikes make up our core anti-tank, with (rend-nerfed, and weapon-gimped) Baals taking the place of anti-horde dakka.
With normal marines, Speeders are a better value, but not for long. The new marine codex will make them more expensive, while making MM bikes cost the same as HB bikes, thus making them a much, much easier choice for those FA slots.