Whoa, whoa, whoa. Just because he doesn't fit the mold of the standard top lane bruiser doesn't mean he's terrible.
You describe the Grab as "clearly useless" and don't even consider the amount of damage he can do grabbing someone (at level 1) into the turret. Hit level 2, and combo it with your knock up for even more damage from the turret.
Most lanes are about 1-hit farming, but plenty of champs also have attack speed boosts. Does having an ability that's useless for farming make a champion terrible? If so, the list of champions you're playing is ridiculously short. Is Warwick a terrible top laner because his W is an attack speed boost (and only AS, no MS)?
Again, the knock up, isn't necessarily about how much burst damage you can deal them. Think about the damage from the extra auto-attacks you, your minions, the turret, and any teammates are going to get on the target while he's knocked up.
The ult is problematic in terms of pushing the lane, but there are other perfectly viable champs in all the lanes that push their lane. Darius's Q? Tristana. Alistar. Soraka. Plenty of other examples.
No gap closer and no retreat? He has a movement speed steroid... and his Rocket Grab is the single best gap closer in the game. Are you kidding me?
Moreover, as I said earlier, there are plenty of other perfectly viable top lane champions that have the same problems you guys are listing for Blitzcrank (no sustain, no good retreat or gapcloser, etc), and some of the problems you list for Blitz just purely are not even actually problems.
If Blitzcrank doesn't have a gap closer, who does?
Heck, just compare Blitz to Darius.
Darius has a pull and a slow. Blitz has a grab with much longer range, and a knock up. Blitz has AS/MS boost, where Darius has an AoE spin. Blitz also has a silence and AoE damage ult (that still does damage while it's passive).
Now, Darius may do more damage than Blitz, and Darius may even beat Blitz in lane, but get to a team fight. Who do you want on your team more? Blitz, who can drop a load of CC on someone (and can probably dish out some good damage if he has top lane farm rather than support) or Darius, who is going to dish out some damage, but mostly stand around stealing kills from your carries?
I don't know how perfectly viable Blitzcrank is, but I think it's definitely worth a shot. It gives you a second support type champion in your team comp for mid and late game. This is the reason I like playing Nunu solotop. I think Nunu is a much better solo-laner (and doesn't need to rely on the turret), but same principle applies, he brings a LOT of mid and late game utility to help your carries win the game.