There are not a lot of really good veteran 3B's out there right now. Other than A-Rod and Aramis Ramirez, neither of which had the remotest chance of going to the Sox, what else was out there? Rolen? Glaus? Beltre? None of them are particularly good options and all of them fall short in a comparison with the balance of power, patience and defense a healthy Lowell provides.
There's more credibility in the argument of going for a better 1B, but again there isn't a lot of guys who would be a better gamble as 1B than Lowell was as a 3B (Lowell has been a better hitter than Beltre each of the last 3 years). Teixeira is just about it, and Teixeira turned them down.
Point is, even if Lowell doesn't emerge as a lineup stalwart this year, that doesn't mean that the signing him was a bad idea. Personally I think he'll be fine in the long run -- he usually is.